BACKGROUND:In 1999, Dr Susan Treloar and colleagues conducted a landmark twin study in Australia and reported their estimate of 51% for the heritability of endometriosis. This important result led several groups to begin mapping genetic factors contributing to increased endometriosis risk. Despite early challenges, advances in genome-wide association studies (GWAS) have identified multiple genetic risk factors and some target genes implicated in follow-up studies on genetic regulation of transcription. Access to large publicly available genetic datasets and analysis with endometriosis GWAS results is also providing new opportunities to answer important questions about comorbid conditions associated with endometriosis and their implications for clinical practice. OBJECTIVE AND RATIONALE:The objective of the review is to summarize the last 25 years of genetic studies in endometriosis, outline contributions to our understanding of the disease, and suggest future directions to accelerate biological insights from genetic studies to improve clinical outcomes. SEARCH METHODS:A comprehensive review of scientific literature on the genetics of endometriosis was conducted through searches in PubMed and Google Scholar up to June 2026. Search terms included "endometriosis AND (genetics OR GWAS OR genetic risk factors)", For studies addressing the functional characterization of genetic risk loci, additional searches employed the terms "endometriosis AND (genotype-phenotype associations OR colocalization OR eQTL OR mQTL OR multi omics methods)". To identify studies examining shared genetic risk between endometriosis and comorbid conditions, the search strategy included "endometriosis AND (genetic correlation OR colocalization OR Mendelian randomisation)". Publications reporting discoveries related to genetic risk factors for endometriosis and studies interpreting their biological and clinical significance were critically evaluated, and 144 publications were discussed in the review. OUTCOMES:Discovery of genetic risk factors started slowly and has accelerated in recent years with developments in technology and international collaborations to combine data and increase statistical power. GWAS have mapped 80 genetic risk factors that implicate gene regulation of hormonal targets, development of the reproductive tract, regulation of cell proliferation, and regulation of epithelial cell differentiation. In common with most other complex diseases, effects of individual common genetic risk factors are small. However, several examples demonstrate that small effect sizes are not a good predictor for the impact of drugs developed against genetically validated targets. Genetic risk factors implicate five genes regulating gonadotrophin release and oestrogen action, the major target pathway of current drugs for treatment of endometriosis demonstrating proof-of-principal for biologically meaningful results. Genetic correlation and Mendelian Randomization studies highlight important causal relationships between endometriosis and comorbid conditions including a possible role for testosterone during development and shared genetic risk factors for gynaecological, gastrointestinal, pain, psychiatric, and inflammatory conditions. Understanding causal relationships between endometriosis and related conditions will aid clinical management and more personalized treatments. WIDER IMPLICATIONS:Genetic studies provide novel insights into endometriosis pathogenesis and associations with related comorbid conditions. Genetic factors modifying gene regulation and disease risk likely act in specific cell types, and access to datasets from genetically informed cell-based models, single-cell and spatial omics data are needed to accelerate progress. Future studies should address critical questions of heterogeneity and disease subtypes, expand the search for genetic risk factors to non-European populations, evaluate the role of rare and structural variants, and better integrate data from functional, genomics, genetics, and clinical studies to reduce diagnostic delay, develop novel treatment strategies, and translate discoveries into personalized management strategies for affected individuals. REGISTRATION NUMBER:N/A.
BACKGROUND:Oocyte donation has been increasingly used in ARTs in recent years. Previous reviews have generally reported positive donor attitudes towards oocyte donation; however, they have not focused on expectations regarding outcomes and potential future contact with donor-conceived offspring. Addressing this gap is critical in light of ongoing ethical, cultural, and legal debates surrounding donor anonymity, financial compensation, and the rights of donor-conceived individuals. OBJECTIVE AND RATIONALE:The systematic review focused on oocyte donors' motivations and expectations on future contact with donor-conceived offspring. This review synthesizes existing empirical evidence to provide a comprehensive overview of donors' perspectives across different sociocultural and legislative contexts. SEARCH METHODS:This systematic review was conducted in accordance with the PRISMA guidelines. A comprehensive search was conducted in May 2024 and updated in September 2025 across PubMed, Embase, PsycINFO, and Web of Science. Search terms were iteratively developed and refined based on preliminary literature reviews and expert consultation. The search strategy identified peer-reviewed empirical research focusing on oocyte donors' attitudes, experiences, or demographic characteristics in any donation context. Grey literature and non-empirical publications were excluded. A total of 37 studies met the inclusion criteria. Data were extracted manually and analysed using a thematic narrative synthesis. Quality appraisal using the Joanna Briggs Institute checklists confirmed that all included studies met minimum methodological quality standards, reducing the risk of significant bias in the synthesis. OUTCOMES:From 9809 records initially identified, and following title and abstract screening, 233 full-text articles were assessed for eligibility. Of these, 37 studies were included. Findings were organized into three overarching themes: motivations for donation, perspectives on anonymity and disclosure, and expectations towards future contact with donor-conceived offspring. Donors were primarily motivated by altruism and financial need, with varying motivations across different sociocultural contexts. Variations were observed in perspectives on anonymity, disclosure, and expectations about future contact, reflecting the influence of legal frameworks, cultural norms, and regulatory backgrounds. Donors in identity-release jurisdictions expressed greater openness to disclosure, while those in anonymous systems emphasized privacy. Although variations in sampling methods, compensation models, and sociocultural contexts may complicate comparisons, the observed patterns were consistent across multiple independent studies, suggesting that these findings are not due to chance but may reflect cross-contextual differences in donor experiences and attitudes. WIDER IMPLICATIONS:This review provides cross-cultural insights into donors' attitudes towards disclosure and future contacts with donor-conceived offspring, highlighting the need for more tailored education and informed consent practices to ensure donors understand the long-term implications of their decisions and receive adequate support throughout the donation process and beyond. For clinicians and policymakers, these results underscore the need to balance donor autonomy and the welfare of donor-conceived families. Further research, particularly longitudinal and cross-jurisdictional studies, is needed to better understand how donors' perspectives evolve over time and to inform evidence-based policy and clinical practice in an era of changing donor anonymity regulations. REGISTRATION NUMBER:CRD42024560246.
BACKGROUND:The global expansion of assisted reproductive technologies (ART), including in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), has raised concerns regarding the impact on placental development and susceptibility to pregnancy complications. Oxygen tension is a central regulator of placental biology, acting both as a physiological developmental signal and, when dysregulated, as a driver of pathology. Early placentation occurs in a naturally low-oxygen environment that is essential for trophoblast specification, invasion, immune-vascular crosstalk, and angiogenesis. These processes are coordinated by hypoxia-inducible factor (HIF) signaling and related oxygen-sensing pathways. OBJECTIVE AND RATIONALE:This review examines the dual role of oxygen in placental development and disease, with particular emphasis on the molecular mechanisms that regulate hypoxia responses during normal and pathological placentation. We further evaluate emerging evidence that ART alters early embryonic and placental programming and propose a mechanistic framework in which ART-induced developmental reprogramming increases susceptibility to hypoxic maladaptation later in pregnancy. SEARCH METHODS:Literature searches were conducted in PubMed and Google Scholar up to June 2026 using combinations of the terms 'hypoxia,' 'placenta,' 'HIF signaling,' 'assisted reproductive technology,' 'in vitro fertilization,' and 'embryo culture.' Studies were limited to English-language publications, with emphasis on human studies and mechanistic investigations in experimental animal and stem cell models. OUTCOMES:Oxygen tension emerges as a master regulator of placental development. Genetic and experimental studies demonstrate that precise temporal and spatial regulation of HIF signaling, including distinct contributions of HIF-1α and HIF-2α, is essential for trophoblast differentiation, placental morphogenesis, vascular development, and fetal viability. Physiological hypoxia orchestrates normal placentation, whereas sustained or dysregulated activation of hypoxia-responsive pathways contributes to an angiogenic imbalance, inflammation, oxidative stress, mitochondrial dysfunction, regulated cell death, and placental insufficiency. Increasing evidence suggests that ART may perturb this tightly regulated developmental program at its earliest stages. Exposure of preimplantation embryos to non-physiological culture conditions can alter their metabolic state, redox homeostasis, epigenetic regulation, and trophectoderm gene expression. These early perturbations may persist into placental development, resulting in impaired trophoblast differentiation and invasion, altered angiogenic signaling, and heightened sensitivity to hypoxic stress. Collectively, findings from experimental models, human placental studies, and emerging single-cell and spatial omics approaches support a model in which ART-induced developmental reprogramming predisposes the placenta to hypoxic maladaptation, thereby increasing susceptibility to disorders such as preeclampsia and fetal growth restriction. WIDER IMPLICATIONS:ART-associated placental hypoxia may originate from altered embryonic programming during preimplantation development rather than arising solely from placental pathology that develops later in pregnancy. Integrating developmental biology, oxygen-sensing pathways, and multi-omic approaches may facilitate the identification of early biomarkers, optimization of embryo culture conditions, and development of targeted strategies to improve maternal and fetal health. REGISTRATION NUMBER:N/A.
BACKGROUND:Female infertility occurs in ∼37% of infertile couples, while premature ovarian insufficiency (POI) impacts 1-3.7% of women under the age of 40. POI is clinically heterogeneous, with various genetic pathways associated with its pathogenesis. Mitochondrial diseases (MDs) are a broad group of clinically heterogeneous genetic conditions characterized by aberrantly functioning mitochondria. MDs have a disproportionate burden on organs and tissues with increased aerobic/energy demands, such as the heart, skeletal muscles, brain, and ovaries. The role of mitochondria in female fertility and ovarian reserve is increasingly being recognized. OBJECTIVE AND RATIONALE:A comprehensive understanding of the role of mitochondria in the maintenance of female fertility is pertinent to better understanding female reproductive potential. In a world with increasing demand for assisted reproductive technologies (ART), due to a considerable rate of global infertility, there is a need to better understand the genes and pathways involved in female reproduction. This review summarizes, evaluates, and explores the current knowledge of mitochondria-associated genes and variants that are implicated in POI, including their function and dysfunction in female reproduction. SEARCH METHODS:We searched articles in the PubMed database, containing the following key words: premature ovarian insufficiency, mitochondria, mitochondrial, premature ovarian failure, genetics, mitochondrial DNA, mtDNA, mitochondrial protein, infertility, premature menopause, mitochondrial donation, assisted reproductive technologies, electron transport chain, oxidative phosphorylation (OXPHOS), mitochondrial disease, oocyte, oogenesis, meiosis, in vitro fertilization, mitoribosome, Perrault syndrome, and ovarioleukodystrophy, in the English-language literature until March 2026. OUTCOMES:Genetic variants that affect mitochondrial genes/proteins can negatively impact ovarian function. Various mitochondrial pathways are associated with female infertility, reflecting the broad sensitivity of ovarian reserve to mitochondrial dysfunction. Mitochondrial dysfunction and infertility can present in isolation or as part of a syndrome. Infertility in women may be the first clinical sign of a MD. Conversely, POI may be an underappreciated symptom of MDs. WIDER IMPLICATIONS:This review draws attention to the fact that females with MDs should be monitored for POI so it can be detected early for prompt and appropriate therapeutic interventions, such as hormone replacement therapy. This is known to mitigate the risk of comorbidities such as cardiovascular and bone disease and will optimize long-term health outcomes. Of equal importance, our review highlights the potential for girls and women presenting with apparently 'isolated' POI to harbour pathogenic variants in MD-associated genes, therefore putting these individuals at risk of developing further clinical manifestations of MDs. We emphasize the need for surveillance in these cases for hearing loss, vision disturbance, cardiomyopathy, muscle weakness and neurodegeneration, depending on the genetic cause. Given that mitochondrial function is essential to female fertility, future therapies for mitochondria-associated infertility could involve mitochondrial supplementation to improve the mitochondrial fraction or mitochondrial donation to optimize the likelihood of reproductive success. Finally, we also discuss the current landscape of biomarkers as potential early diagnostic tools for POI. Whilst currently rudimentary in their clinical utility, the further development of early screening methods will be invaluable for the detection, diagnosis, and early intervention of POI. REGISTRATION NUMBER:N/A.
BACKGROUND:Peritoneal adhesions are a major and often under-recognized cause of morbidity following abdominopelvic inflammation and surgery, contributing to chronic pelvic pain, infertility, bowel obstruction, and surgical complications. Endometriosis, affecting 10-11% of reproductive-age women, is strongly associated with adhesions; however, whether adhesion formation in endometriosis reflects general peritoneal injury or disease-specific mechanisms remains unclear. OBJECTIVE AND RATIONALE:This narrative review examines (i) physiological peritoneal healing and mechanisms of adhesion formation, (ii) how the altered peritoneal microenvironment in endometriosis may modify adhesiogenesis, and (iii) current and theoretical therapeutic and preventative strategies for adhesions in endometriosis. This will improve current understanding of adhesiogenesis and the implications of adhesions in women with endometriosis, and will identify areas for future research to reduce morbidity in this patient population. SEARCH METHODS:Literature searches were performed in PubMed, Embase and Cochrane Library using combinations of the terms 'endometriosis', 'adhesions', 'pelvic', 'abdominal', 'peritoneal' between January 2000 and February 2026. Randomized-controlled trials, observational studies, systematic reviews, and relevant experimental studies were included. Case reports and non-English publications were excluded. A total of 124 studies were included in this literature review. OUTCOMES:Adhesion formation results from dysregulated peritoneal repair involving inflammation, coagulation-fibrinolysis imbalance, and fibroproliferative remodelling. Evidence suggests that endometriosis creates a peritoneal environment characterized by chronic inflammation, altered immune responses, repeated tissue injury, and exposure to menstrual debris and iron, which may promote adhesiogenesis. However, much mechanistic understanding is derived from experimental models, and direct evidence from patients remains limited. Prevention of adhesiogenesis in endometriosis currently focuses on intra-operative interventions to limit adhesion development following endometriosis-related surgery, rather than preventing disease-related adhesiogenesis in the absence of surgery. We discuss potential pharmacotherapies, including hormonal treatments, identified by experimental studies, which may perturb endometriosis-related adhesiogenesis. WIDER IMPLICATIONS:Current data support a model in which adhesion biology in endometriosis largely reflects conserved wound-healing pathways but develop within a chronically altered peritoneal microenvironment that lowers the threshold for adhesiogenesis and promotes persistence and recurrence. Clarifying endometriosis-specific drivers and generating robust human data are essential to develop targeted adhesion-prevention strategies and improve long-term outcomes for affected patients. Notably, there are currently no effective pharmacological therapies to prevent or treat adhesions, representing a major unmet clinical need in endometriosis care. REGISTRATION NUMBER:N/A.
BACKGROUND:The foetal testes produce the androgens necessary to masculinise the developing embryo and support the maturation of germ cells, that will eventually develop into sperm, thus ensuring future reproductive capacity. The testes develop from the bi-potential gonads in a highly orchestrated process resulting in the differentiation of a complex tissue with multiple cellular lineages. While recent transcriptomic and chromatin-based analyses of human foetal testes have provided an unprecedented level of insight into signalling pathways activated during this process, proteomic studies of the human foetal gonads remain limited. Proteins are active molecules and post-translational modification (PTM) of proteins influences protein activity, stability and localisation. Studies have shown that PTMs regulate critical proteins in testis development, and their disruptions are implicated in congenital disorders including differences of sex development (DSD), in which sex development is atypical. Despite this, the role and regulation of protein PTM during human testis development remains poorly understood due to limited access to human foetal gonadal tissue, a paucity of large-scale proteomics studies, and a lack of robust of human gonad in vitro models. OBJECTIVE AND RATIONALE:This review aims to provide a comprehensive analysis of validated PTMs affecting proteins critical for testicular development. We discuss PTMs with evidence for a role in normal testis development, and highlight those disrupted in DSD. We review emerging techniques, including proteomic technologies and organ modelling systems that may advance our understanding of PTMs in foetal testis development. We discuss challenges that have restricted the application of these technologies and how overcoming these will significantly improve our understanding of testis development and disease, diagnostics and patient outcomes. SEARCH METHODS:We searched PubMed and the University of Melbourne library for peer-reviewed English-language studies using keywords such as phosphorylation, SUMOylation, acetylation, ubiquitination alongside each protein of interest. PTM sites in proteins involved in testis development were identified using the PhosphoSitePlus database focusing those confirmed in in vitro or animal model studies. ClinVar and the Human Gene Mutation Database were used to identify patient variants that may disrupt PTM sites. OUTCOMES:Our review finds that proteins required for human foetal testis development are subject to extensive PTM. Several PTM sites and PTM-mediated pathways [e.g. MAPK (mitogen-activated protein kinase) pathway] are disrupted in patients with DSD or related conditions. While recent advances in proteomics technologies hold considerable promise, their application to human foetal gonads has been constrained by technical, ethical, and logistical challenges. Encouragingly, emerging high-sensitivity and low-input technologies, alongside stem cell-based approaches, offer viable pathways to overcoming these barriers. WIDER IMPLICATIONS:The relationship between gene regulation, protein expression, and cellular outcome is inherently non-linear, shaped by additional regulatory layers-most notably PTMs. The contribution of PTMs to human testis development in both typical and atypical contexts is a major knowledge gap. Addressing this gap has broad clinical and biological relevance: it may help improve genetic diagnosis or shed light on how proteins or pathways critical for testis development respond to environmental signals-an increasingly pressing question as declining global fertility rates bring testicular function under greater scrutiny. REGISTRATION NUMBER:N/A.
Abstract BACKGROUND Extended embryo culture beyond Day-6 remains controversial. While a small subset of embryos reaches the blastocyst stage on Day-7, their developmental competence and reproductive potential remain uncertain. Data from previous studies have been limited and heterogeneous, particularly for meta-analytical evaluation of euploid embryo transfers. OBJECTIVE AND RATIONALE This systematic review and meta-analysis compared clinical pregnancy (CPR) and live birth rates (LBR), as well as implantation (IR), miscarriage (MR), and ongoing pregnancy rates (OPR), between Day-7 frozen blastocysts and those obtained on Day-5 and/or Day-6. Euploidy rates by day of blastocyst development and available perinatal outcomes were also assessed. SEARCH METHODS Conducted according to PRISMA guidelines, this review included full-length English-language human studies, published up to 1 April 2026, comparing frozen transfers of Day-7 versus Day-5/6 blastocysts and reporting at least clinical pregnancy and/or live birth outcomes. Random-effects models were used to estimate pooled odds ratios (ORs) with 95% confidence intervals (95% CI). Obstetric and perinatal outcomes were analysed when available. OUTCOMES A total of 17 retrospective studies were included. Across 2908 Day-7 transfers, CPR and LBR were significantly lower than for Day-5 (CPR: OR 0.26, 95% CI 0.19–0.35, I2: 76.2%, 16 studies, 2799 and 34 442 transfers at Day-7 and Day-5, respectively; LBR: OR 0.23, 95% CI 0.16–0.33, I2: 76.3%, 12 studies, 2687 and 31 630 transfers at Day-7 and Day-5, respectively) and Day-6 blastocysts (CPR: OR 0.38, 95% CI 0.31–0.48, I2: 54.4%, 16 studies, 2799 and 28 563 transfers at Day-7 and Day-6, respectively; LBR: OR 0.37, 95% CI 0.28–0.49, I2: 60.2%, 12 studies, 2687 and 26 687 transfers at Day-7 and Day-6, respectively). Similarly, IR and OPR were significantly lower in the Day-7 group than in the Day-5 or Day-6 groups, while MR was higher compared with Day-5 blastocysts (OR 1.54, 95% CI 1.04–2.30, I2: 51.1%, 14 studies, 685 and 18 609 pregnancies at Day-7 and Day-5, respectively). When considering only PGT-A euploid blastocysts, CPR and LBR remained significantly lower in the Day-7 group compared with Day-5 (CPR: OR 0.21, 95% CI 0.14–0.33, I2: 58.0%, 7 studies, 365 and 7064 transfers at Day-7 and Day-5, respectively; LBR: OR 0.21, 95% CI 0.17–0.28, I2: 0%, 6 studies, 364 and 5924 transfers at Day-7 and Day-5, respectively) and Day-6 (CPR: OR 0.36, 95% CI 0.28–0.46, I2: 40.8%, 7 studies, 365 and 4617 transfers at Day-7 and Day-6, respectively; LBR: OR 0.36, 95% CI 0.28–0.46, I2: 0%, 6 studies, 364 and 3782 transfers at Day-7 and Day-6, respectively), and MR was higher than Day-5 (OR 2.78, 95% CI 1.57–4.89, I2: 32.4%, 6 studies, 95 and 4018 pregnancies at Day-7 and Day-5, respectively). Euploid rates were also significantly reduced for Day-7 blastocysts compared with Day-5 and Day-6. Data on perinatal outcomes were limited but overall reassuring. The pooled analyses revealed low to high heterogeneity across studies. WIDER IMPLICATIONS Despite lower success rates, including for euploid embryos, Day-7 blastocysts may still contribute to cumulative live birth rates, notably in poor-prognosis patients. Prospective studies are needed to better assess perinatal and long-term outcomes. REGISTRATION NUMBER PROSPERO (CRD420251114251)
BACKGROUND:With growing interest in ART, fertility preservation, and postmenopausal health of women, reproductive medicine is increasingly focused on characterizing oocytes and ovarian tissue composition, as well as understanding the molecular mechanisms that guide ovarian function throughout its lifecycle. High-throughput omics technologies have enabled the characterization of different molecular layers, leading to substantial advances in our understanding of their complex dynamics. However, not all molecular aspects are studied equally, and studies examining the same modalities often show inconsistencies, underscoring the need for data standardization and highlighting the potential for using transformative artificial intelligence and machine-learning (AI/ML) methods for ovary studies. OBJECTIVE AND RATIONALE:This study aims to evaluate how multi-omic studies have advanced our understanding of the ovarian lifecycle from fetal development to postmenopause. We systematically reviewed published studies that have investigated molecular/omic layers, including the genome, methylome, transcriptome, and proteome throughout ovarian development and aging. Our analysis identified key molecular and cellular patterns, highlighted inconsistencies across studies and addressed gaps in data analysis, interpretation, and reproducibility to guide future research. SEARCH METHODS:We conducted a systematic literature search of Medline (PubMed), Embase (Ovid), and Web of Science Core Collection (Clarivate) using a combination of controlled and free text terms for human ovary, oogenesis, folliculogenesis, ovary development and (epi)genome, transcriptome, proteome, and multi-omic mechanisms to find relevant articles published before August 2025. To focus the scope of the current review, studies of domesticated and farm animals, rodents and other model organisms, non-human primates, as well as those examining various human ovarian pathologies were excluded. OUTCOMES:The search identified 23 546 studies for screening, of which 637 full-text studies were assessed for eligibility. Subsequently, we extracted data from 121 studies. Most studies analyzed the transcriptome of oocytes, granulosa cells, and ovarian tissue from reproductive-age individuals (n = 91), with fewer studies examining samples from individuals of advanced reproductive age (n = 45) and fetal (n = 16) samples. Transcriptome analyses were most common (n = 103, 85%), followed by proteome (n = 19, 16%) and epigenome (n = 14, 12%) studies. We found substantial variation in how studies defined and reported participants' groups as well as in their sequencing technologies and data analysis methods, with a lack of standardized reporting of background clinical information, data analysis methods, and pipeline details. The key findings underscore the prevailing consensus on genes defining major ovarian cell types and their roles throughout the ovarian lifespan, from prenatal development to postmenopausal transformation. This review highlighted the underrepresentation of certain patient groups, particularly prepubertal and peri-/postmenopausal individuals, among researched populations, due to obvious clinical and ethical reasons. WIDER IMPLICATIONS:This scoping review offers a comprehensive overview and benchmark of the current state of high-throughput omics-based research on ovarian cellular composition and molecular dynamics. To address these shortcomings, we propose general recommendations for multi-omics ovary studies and emphasize the necessity for more thorough multi-omic data integration by effectively applying novel AI/ML approaches. They can potentially improve the quality of multi-omics analyses at both single-cell and tissue levels despite limited sample sizes and enable integration of molecular profiling data with clinical and radiology datasets, enabling a more comprehensive understanding of ovarian biology. Such advancements can enhance reproducibility of research findings and guide future research to deepen our understanding of ovarian biology and ultimately support the development of medical technologies for better preserving fertility and alleviating infertility. REGISTRATION NUMBER:A protocol was published a priori on the Open Science Framework (https://osf.io/z38gb/).
BACKGROUND:During routine fertilization checks, the presence of two pronuclei (2PN) with two polar bodies is generally regarded as normal fertilization. Nonpronuclear (0PN) zygotes often suggest failed fertilization, but some do cleave and develop into viable embryos due to delayed appearances of pronuclei that were originally missed. Monopronuclear (1PN) zygotes may result from a lack of maternal or paternal pronuclei, fusion of pronuclei, or asynchronous appearance of pronuclei. These embryos may sometimes be transferred when no others are available. OBJECTIVE AND RATIONALE:Current evidence on the euploidy rates (ER) and clinical outcomes of embryos derived from 0PN and 1PN zygotes is discordant across publications, resulting in conflicting conclusions regarding whether they are suitable for transfer. This review aims to present an updated synthesis of current knowledge regarding the potential adverse outcomes associated with 0PN and 1PN embryo transfers and to identify important gaps that should be addressed in future studies. SEARCH METHODS:CENTRAL, Embase, Fertility and Sterility, MEDLINE, and PubMed databases were searched on 10 November 2025 using the keywords '0PN embryo' and '1PN embryo'. All types of studies, except case reports and reviews, were included if they compared 0PN/1PN with 2PN zygotes in women of any age undergoing IVF/ICSI cycles, followed by single embryo transfer (SET) and/or preimplantation genetic testing for aneuploidy (PGT-A). Pronuclear status was assessed 16-18 h post insemination. Only data from SET cycles were included for embryo transfer outcomes. OUTCOMES:Eleven studies reporting on 37 240 SET cycles and 27 195 biopsied blastocysts were included. Compared to 2PN embryos, those derived from 0PN zygotes showed no significant differences in live birth rate (LBR) (OR = 0.94, 95% CI = 0.83-1.06, P = 0.33, I2 = 2%), clinical pregnancy rate (CPR) (OR = 0.96, 95% CI = 0.85-1.08, P = 0.48, I2 = 0%), miscarriage rate (MR) (OR = 1.04, 95% CI = 0.84-1.29, P = 0.70, I2 = 0%), malformation rate (OR = 1.05, 95% CI = 0.45-2.45, P = 0.91, I2 = 0%), and ER (OR = 0.86, 95% CI = 0.50-1.48, P = 0.59, I2=63%). However 0PN-derived embryos had a significantly lower blastulation rate (OR = 0.34, 95% CI = 0.17-0.68, P = 0.002; I2 = 99%). Compared to 2PN, 1PN embryos showed a significantly lower LBR (OR = 0.83, 95% CI = 0.70-0.98, P = 0.03, I2 = 12%), CPR (OR = 0.81, 95% CI = 0.68-0.96, P = 0.01, I2 = 22%), and blastulation rate (OR = 0.26, 95% CI = 0.14-0.48, P < 0.001; I2 = 96%), but a similar MR (OR = 1.34, 95% CI = 0.76-2.37, P = 0.31, I2 = 67%), malformation rate (OR = 0.46, 95% CI = 0.16-1.32, P = 0.15, I2 = 0%), and ER (OR = 1.05, 95% CI = 0.70-1.58, P = 0.80, I2 = 66%). WIDER IMPLICATIONS:In the meta-analysis, both 0PN- and 1PN-derived embryos showed no significant differences in rates of euploidy, miscarriage, and malformations compared to 2PN-derived embryos. However, clinical pregnancy and LBRs were significantly lower for 1PN, but not 0PN, embryos than for 2PN embryos. These findings suggest that 0PN-, and possibly even 1PN-, derived embryos, especially those that have undergone PGT-A, may represent a viable source for embryo transfer in certain clinical settings, particularly in cases where 2PN embryos are limited or unavailable. REGISTRATION NUMBER:CRD42024546685.
BACKGROUND:Sperm mitochondrial DNA copy number (mtDNAcn) has emerged as a promising biomarker of sperm health, providing molecular insight beyond what is captured by standard semen analysis. Elevated sperm mtDNAcn has been consistently associated with lower sperm motility, concentration, and morphology, as well as prolonged time-to-pregnancy in natural conception and reduced fertilization potential in ART. OBJECTIVE AND RATIONALE:This review summarizes current evidence on the biological underpinnings of sperm mtDNAcn, including its regulation during spermatogenesis, the role of nuclear-encoded mitochondrial proteins such as TFAM (mitochondrial transcription factor A), and its potential epigenetic modulation through sperm DNA methylation. We evaluate general population and clinic-based studies linking sperm mtDNAcn to semen quality, couple fecundity, and early embryo development, while highlighting methodological considerations such as quantification techniques and somatic cell contamination. SEARCH METHODS:A literature search was conducted to identify human studies evaluating sperm mtDNAcn in relation to male fertility, semen quality, reproductive outcomes, and embryology outcomes, as well as experimental models investigating the underlying biological mechanisms of mtDNA regulation during spermatogenesis via TFAM up to 1 April 2026. Searches were performed in PubMed, Web of Science, and Scopus using combinations of keywords and Medical Subject Headings (MeSH), including sperm mitochondrial DNA copy number, mtDNAcn, male infertility, pregnancy outcomes, ART outcomes, semen quality, sperm epigenetics, TFAM, mitochondrial transcription factor A, and sperm mtDNA regulation. Reference lists of relevant reviews and primary articles were manually screened to identify additional studies. Eligible studies included observational epidemiologic studies, clinical cohort studies, and experimental investigations that quantified sperm mtDNAcn and examined associations with semen parameters, fertility outcomes, or sperm epigenetics. No restrictions were placed on geographic location, while only articles published in English were considered. OUTCOMES:Across 21 epidemiologic, experimental, and clinical studies, elevated sperm mtDNAcn has been consistently associated with reduced sperm quality and inconsistently associated with diminished couple-level reproductive potential. Higher mtDNAcn is associated with lower sperm concentration, total sperm count, motility, and normal morphology, as well as higher sperm DNA fragmentation and chromatin abnormalities. It has also been linked to reduced likelihood of pregnancy and poorer embryo quality. Emerging evidence indicates associations between sperm mtDNAcn and altered nuclear DNA methylation patterns, supporting a role for mitochondrial-nuclear crosstalk. Collectively, these findings position sperm mtDNAcn as a biologically informative and clinically relevant indicator of male reproductive health that may complement, or potentially enhance, traditional semen analysis in both research and clinical settings. WIDER IMPLICATIONS:Sperm mtDNAcn holds promise as a biomarker for male fertility assessment, yet its full clinical potential has not been yet to be realized. Establishing standardized measurement protocols across sperm fractions and laboratory platforms will be essential for enabling cross-study comparability and facilitating clinical translation. Large prospective studies are needed to define clinically meaningful thresholds and better characterize the relationship between mtDNAcn alterations and spermatogenic impairment. Intervention strategies targeting mtDNA biogenesis and depletion, including antioxidant strategies and mitochondria-directed pharmacotherapy, warrant further investigation in the context of male fertility and fecundity. Collectively, sperm mtDNAcn may serve as an adjunctive marker in the assessment of male reproductive health and may inform future precision medicine approaches. REGISTRATION NUMBER:N/A.
BACKGROUND:Over the past decade, extracellular vesicles (EVs) released from embryos have received increasing attention for their potential role in influencing embryo-maternal communication events during implantation. Several studies have explored the relationship between embryo-derived EVs and embryo developmental competence. To synthesize current insights, we conducted a systematic review of the literature, highlighting key findings and evaluating how embryo EV profiles relate to embryo developmental competence. OBJECTIVE AND RATIONALE:A systematic review was conducted in accordance with the PRISMA guidelines. The review intended to (i) synthesize the existing knowledge on embryo-derived EV concentration, size, and cargo and their association with embryo developmental competence in mammalian systems, (ii) propose recommendations for future research, and (iii) assess the potential impact of the current literature on embryo selection during IVF. SEARCH METHODS:A literature search was performed across five databases (MEDLINE, EMBASE, BIOSIS Previews, Web of Science, and Scopus), involving keywords 'Embryo' AND 'Extracellular vesicle'; many synonyms were included for each keyword along with map terms when available. The search was conducted on 14 January 2026. References were screened for additional relevant records. Abstract-only authors were contacted to determine whether full-text was available. The inclusion criteria were 'collection of culture media from embryos of any mammalian species and isolation and assessment of EVs from it'. Reasons for exclusion included: 'No collection of embryo culture media', 'No characterisation or assessment of EVs', 'Not embryo-derived EVs', and 'Review/Theory/Lacking primary data'. OUTCOMES:Our searches collected a total of 15 620 records; of these, 32 studies from 34 records were included in the systematic review. The included studies were divided into two groups: (i) studies of embryonic EV characteristics and (ii) studies that investigated the association of embryonic EV characteristics and embryo developmental competence, including embryo quality assessments based on morphology, ploidy, and pregnancy. Wide variations in embryo EV concentration and size were reported, largely due to differences in study designs. Overall, the quality of the existing literature ranged from low to moderate quality. Among the three human and eight bovine studies that examined the association of embryo EV concentration, size, and/or cargo with developmental competence, the certainty of the evidence for all outcomes was consistently very low. Studies often reported opposing directions of effects, whilst others showed no clear association. Inconsistencies in EV isolation techniques, timing of EV sampling, and methods for embryo quality assessments complicate efforts to assess whether embryonic EVs can be used as a reliable biomarker. WIDER IMPLICATIONS:Current evidence does not support the use of embryo EVs as biomarkers during IVF procedures. To enhance reproducibility and the production of high-quality studies, it is recommended that studies incorporate standardized approaches to EV isolation and characterization to align with best practices. In addition, the timing of EV sampling should be standardized, appropriate media controls used, and optimal measures of developmental competence, such as live birth, reported. Adherence to these recommendations will enable future studies to build constructively on the existing literature, with the possibility of informing clinical applications that improve patient outcomes. REGISTRATION NUMBER:https://doi.org/10.17605/OSF.IO/TPXVU.
BACKGROUND:Oocyte donation (OD) is an established ART involving an oocyte donor and recipient with a rising number of treatments. Previous meta-analyses highlight increased risks of hypertensive complications compared to naturally conceived (NC) and IVF/ICSI pregnancies, including pregnancy-induced hypertension (PIH) and preeclampsia (PE), but limitations exist due to study quality and heterogeneity. OBJECTIVE AND RATIONALE:The DONOR (DONation of Oocytes in Reproduction) individual participant data (IPD) meta-analysis aims to generate clinically relevant and robust evidence regarding the development of hypertensive complications in OD pregnancies compared to autologous pregnancies. IPD meta-analyses offer an advantage over current meta-analyses, as bias is reduced by using IPD of original studies, allowing reliability checks, correction for confounders, and examining causes of heterogeneity by subgroup analyses. Furthermore, using IPD increases statistical power and generalizability of results. SEARCH METHODS:A literature search was conducted using PubMed, EMBASE, and Cochrane up to March 2024, with a last update performed in February 2025. We included observational studies that compared a cohort of women pregnant after OD beyond 20 weeks of gestation with an autologous pregnancy cohort (NC or IVF/ICSI), and reported on hypertensive pregnancy complications. Risk of bias was assessed using the ROBINS-I tool. Authors of eligible articles were invited to share IPD. The DONOR IPD meta-analyses were executed using both a one- and two-stage approach, adjusted for maternal age, parity, and multiple gestation. Furthermore, sensitivity, meta-regression and subgroup analysis were performed. OUTCOMES:IPD was requested for 48 cohorts, and provided from 16 cohorts with data of 2747 OD, 4699 IVF/ICSI, and 33 323 NC pregnancies. The one- and two-stage approach comparing OD to autologous pregnancies showed adjusted ORs of respectively 2.62 (95% CI 2.22-3.10) and 2.85 (95% CI 2.30-3.54; I2 44%; moderate certainty) for hypertensive complications in total, 2.15 (95% CI 1.73-2.68) and 1.49 (95% CI 0.80-2.80; I2 74%; low certainty) for PIH, and 2.28 (95% CI 1.88-2.78) and 2.39 (95% CI 1.94-2.94; I2 0%; high certainty) for PE. When the autologous group was split into NC and IVF/ICSI pregnancies, higher risks for hypertensive complications, including PIH and PE, persisted in the OD group. The results of the IPD meta-analyses for HELLP syndrome show a higher risk in OD pregnancy, though with a broad 95% CI. Sensitivity meta-analyses for risk of bias showed comparable results. Subgroup analyses indicated increased risks for hypertensive complications in OD pregnancy, regardless of maternal age, BMI, multiple pregnancy, parity, ethnicity, medical history, and number of transferred embryos. A potential lower risk for hypertensive complications was found when acetylsalicylic acid or heparin is used during OD pregnancy compared to both autologous and NC pregnancy. WIDER IMPLICATIONS:The DONOR IPD meta-analysis provided a unique opportunity to assess the risk for hypertensive complications in OD compared to autologous pregnancy. The results must increase alertness of health care professionals who are involved in OD health care towards the risk profile of these pregnancies, as the DONOR IPD meta-analysis results in the best evidence-based statement for international guidelines in obstetrics to date. Possibly, preventive treatment with low-dose acetylsalicylic acid is successful in lowering the risk for hypertensive complications, though more evidence is needed to confirm this effect, alongside the underlying pathological mechanism. REGISTRATION NUMBER:CRD42021267908.
BACKGROUND:Preconception health (PCH) is a globally accepted strategy to reduce preventable adverse pregnancy outcomes and ultimately improve the health of the unborn child. Optimal PCH can be achieved through preconception care (PCC), which encompasses the behavioural, biomedical, and social interventions women and couples undertake and/or receive before conception. However, there is a lack of clarity on various aspects of PCH and PCC, such as what constitutes preconception risk factors, what the optimal interventions are, when the preconception period is, and who the overall target population is. Additionally, marginalised groups such as sexual and racial or ethnic minority individuals are routinely excluded from PCH research and PCC interventions. PCH and PCC are topical issues given changing societal norms worldwide, such as delayed childbirth, exponential rises in fertility treatments, and the growing trend of unplanned pregnancies. We hypothesised that the ambiguity surrounding the definition of PCH and PCC may limit their understanding and application to improve pregnancy and childhood outcomes. OBJECTIVE AND RATIONALE:This is a systematic review of existing definitions of PCH and PCC to understand the commonalities and disparities in definitions and critical components of PCH and PCC, to aid in the development of a comprehensive and globally standardised definition. SEARCH METHODS:MEDLINE, PubMed, EMBASE, Cochrane Library, CINAHL, Google Scholar, PsychINFO, and Google were searched to identify published studies, guidelines, and public health websites containing definitions of PCH and PCC published between January 1993 and October 2024. No restrictions were placed on language. We searched academic databases, organisational reports, and policy documents to capture the full range of definitions across clinical, health, and policy contexts. OUTCOMES:The narrative synthesis of 176 publications showed heterogeneity in the definitions of PCH and PCC. The themes developed from the thematic analysis showed that PCC is preventative care which identifies and utilises interventions to manage individuals' preconception risk factors and aims to improve pregnancy outcomes by optimising the short- and long-term health of potential parents and their children. The analysis also showed that PCH is relevant across the entire reproductive lifespan. PCC was described as a continuum of care that occurs before conception and encompasses the health of all potential parents, not just women. WIDER IMPLICATIONS:This systematic review found there is a lack of universality in the definitions of PCH and PCC. Current definitions often narrowly focus on women planning pregnancy, which may exclude important demographics such as unintended pregnancies and fathers, and aligned health needs such as contraception in the preconception period. We propose that there is a need for a definition that captures various demographics and emphasises a life-course approach to reproductive health, acknowledging that the preconception period is much wider than only the period in which couples are actively trying to conceive. Congruence between policymakers, researchers, and public health professionals on the definition of PCH and PCC may address research operationalisation and clinical implementation to better assess global uptake and impact. REGISTRATION NUMBER:CRD42023480536.
BACKGROUND:Male fertility investigation is currently limited to semen analysis. However, the origins of abnormal sperm parameters are not well-understood, and normal sperm do not assure fertility in men. Improved pathophysiological and prognostic insight might be achieved utilising additional measures of male reproductive tract function. Cytokine and chemokine levels in seminal plasma (SP) may be relevant, but evidence on their clinical significance is unclear. The utility of measuring SP cytokines remains uncertain, and no consensus exists on which cytokines are most informative. OBJECTIVE AND RATIONALE:To perform a systematic review and meta-analysis on the association between fertility status and concentration of seminal plasma cytokines in men. The sixth edition of the WHO laboratory manual for the examination and processing of human semen raises the prospect of evaluating cytokines in SP as part of an extended examination. We performed a systematic search and meta-analysis to assess whether the current literature is sufficient to identify cytokines present in human SP that exhibit a relationship with fertility status in men. SEARCH METHODS:We searched PubMed, Web of Science, Scopus, and Embase from inception until April 2025, using keywords pertaining to seminal fluid and cytokines, restricted to humans and the English language. Original data with values reported as concentration of cytokines in SP of men clearly defined as infertile, compared to a discernible population of fertile/normozoospermic healthy control men, were included. A total of 5737 studies were identified, with 2737 duplicates removed. Title and abstract screening were performed for 3000 studies, then 291 studies underwent full-text screening, and 68 studies progressed to quality assessment using the NHLBI-NIH quality assessment tool and 52 studies underwent data extraction. OUTCOMES:We identified 52 research articles published from 1993 to 2025 that quantified at least one cytokine in the seminal plasma of 8153 men, after 19 studies of poor quality and/or containing serious flaws were excluded. Data on 30 cytokines in the SP of healthy control and infertile men were extracted and included in narrative synthesis. Compared to fertile controls, infertile men had elevated concentrations of IL6 (SMD 0.39, 95% CI; 0.14-0.64, I2 = 80.7%), TNFA (SMD 0.13, 95% CI; 0.00-0.25, I2 = 4.3%), and CXCL8 (SMD 0.24, 95% CI; 0.06-0.43, I2 = 0.0%) in seminal plasma. IL6 reported a high degree of heterogeneity between studies, whilst CXCL8 and TNFA reported low heterogeneity. No significant moderator effects due to study quality or composition of the control cohort were identified. WIDER IMPLICATIONS:With one in six couples experiencing infertility worldwide and a male factor identified as a primary or contributing cause in up to 50% of cases, there is a strong imperative to develop a better understanding of the pathophysiology of male infertility. This analysis identifies an association between infertility and elevated SP IL6, CXCL8, and TNFA, but whether these pro-inflammatory cytokines reflect events affecting fertility or are simply markers of transient inflammation and/or cofactors of fertility status remains to be determined. Greater precision in quantifying seminal plasma cytokines will be attained, and additional informative cytokines may be identified, by utilising standardised technical approaches in future studies. REGISTRATION NUMBER:CRD42023398438.
BACKGROUND:Tryptophan metabolism within the placenta generates bioactive metabolites, including serotonin (5-hydroxytryptamine; 5-HT), melatonin, and kynurenine derivatives, that regulate immune tolerance, vascular function, oxidative balance, and fetal neurodevelopment. Increasing evidence indicates that placental handling of tryptophan is dynamically regulated across gestation and is highly sensitive to maternal environmental and metabolic cues. OBJECTIVE AND RATIONALE:The aim of this review is to examine placental tryptophan metabolism across gestation, with a focus on the 5-HT, melatonin, and kynurenine pathways. We address how these pathways are regulated during normal pregnancy and how maternal factors, including inflammation, hypoxia, oxidative stress, and cardiometabolic dysfunction, influence placental tryptophan handling in pregnancy complications such as early pregnancy loss, preeclampsia, fetal growth restriction, and preterm birth. SEARCH METHODS:PubMed was searched using predefined terms related to placental tryptophan metabolism, 5-HT, melatonin, kynurenine, fetal programming, neurodevelopment, and pregnancy complications. Only full-text, peer-reviewed articles published in English were included. Abstracts and conference proceedings were excluded due to their limited data reliability. OUTCOMES:Placental tryptophan metabolism shows clear gestational stage-dependent regulation, and early pregnancy emerges as a formative period when pathway activity and metabolite balance are first established. From early pregnancy, maternal-decidual kynurenine pathway activity and placental 5-HT synthesis intersect with immune tolerance, vascular adaptation, and neurodevelopmental signaling. Across gestation, maternal inflammation, hypoxia, oxidative stress, and cardiometabolic disturbance can redirect the tryptophan flux and shift the balance between 5-HT/melatonin and downstream kynurenine metabolites. Evidence across pregnancy complications links early pathway disruption to pregnancy loss and supports the view that early metabolic perturbations contribute to vulnerability for later placental dysfunction, including preeclampsia, fetal growth restriction, and preterm birth. WIDER IMPLICATIONS:Placental tryptophan metabolism changes across gestation, making early pregnancy a critical window when pathway balance and fetal exposure to neuroactive metabolites are first set. Maternal inflammation, metabolic status, nutrition, and drug exposures may alter this balance, with the placenta acting as the key interface that transmits maternal signals to the fetus and shapes neurodevelopmental trajectories. To define the clinical relevance of altered tryptophan catabolism, longitudinal human studies are needed to link placental phenotypes with pregnancy outcomes and postnatal neurodevelopment. These should be complemented by mechanistic models that resolve regulation in early gestation. REGISTRATION NUMBER:n/a.
BACKGROUND:The optimal endometrial preparation protocol for frozen embryo transfer (FET) remains a subject of ongoing investigation. HRT is the most commonly used approach, but natural cycle (NC) FET has regained attention due to potential improvements in maternal and perinatal outcomes. Despite growing observational evidence supporting NC FET, its adoption is limited by logistical challenges in cycle monitoring and scheduling. Recently, the natural proliferative phase (NPP) FET protocol has been introduced, combining the physiological benefits of a functional corpus luteum with greater scheduling flexibility. OBJECTIVE AND RATIONALE:Previous systematic reviews have largely focused on luteal phase support (LPS) or have provided narrative summaries susceptible to selection bias. This systematic review aimed to evaluate the impact of different execution strategies on reproductive outcomes across true-NC and modified-NC and to compare NPP FET with other protocols. SEARCH METHODS:A comprehensive search of MEDLINE, Embase, Global Health, and Cochrane Library was conducted from database inception to 10 November 2024. The search included keywords such as 'frozen embryo transfer', 'natural cycle', 'pregnancy', 'live birth', and 'delivery' with no language or filter restrictions. Reference lists of included studies were screened to identify additional relevant studies. OUTCOMES:A total of 70 studies were included: 8 randomized controlled trials (1 with low risk of bias and 7 with some concerns), 16 non-randomized interventional studies (with risk of bias being moderate for 4, serious for 6, and critical for another 6), and 46 observational studies (80.4% of which were good quality) assessing prognostic factors. In true-NC FET, prolonged follicular phases did not adversely affect outcomes. Ovulatory cycles were associated with significantly higher live birth rates (LBRs) than cycles with luteinized unruptured follicle (risk ratio (RR): 1.16, 95% CI: 1.04-1.29, I2 = 0%, three studies, 2907 cycles, very low-certainty evidence). Despite variability in ovulation timing methods, performing FET on serum LH surge +6 to +7 days yielded comparable reproductive outcomes. In modified-NC FET, two observational studies reported similar LBRs when triggering ovulation at follicle diameters between 13 and 22 mm, provided the endometrial thickness was >7 mm and serum progesterone was below 1.5 ng/ml. LPS with vaginal progesterone improved LBRs in true-NC compared to no LPS (RR: 1.43, 95% CI: 1.16-1.78, I2 = 0%, 923 cycles, two studies, moderate-certainty evidence), but showed no benefit in modified-NC FET (RR: 1.04, 95% CI: 0.82-1.32, I2 = 0%, 667 cycles, two studies, moderate-certainty evidence). In NPP FET, a meta-analysis showed higher LBRs compared to HRT FET (RR: 1.25, 95% CI: 1.13-1.38, I2 = 5.36%, 3397 cycles, three studies, very low-certainty evidence). WIDER IMPLICATIONS:NC FET protocols are associated with operational constraints. Modified-NC and NPP strategies may balance physiological benefits with scheduling convenience, though high-quality evidence remains limited. Routine LPS improves LBRs in true-NC FET but not in modified-NC. The apparent equivalence of outcomes when hCG is administered across a follicle size range of 13-22 mm suggests a potential 7-day window of scheduling flexibility for modified-NC FET; however, this finding warrants further validation. NPP FET may offer enhanced scheduling convenience without compromising reproductive outcomes, but warrants additional investigation through high-quality trials. REGISTRATION NUMBER:PROSPERO: CRD42023385304.
BACKGROUND:Understanding the mechanisms that promote or hinder healthy placental development and functionality is fundamental to advancing the field of fetal and reproductive medicine. Syncytiotrophoblast (STB) are highly specialized trophoblast which develop and gain functional maturity during the first trimester of pregnancy. STB are critical to many placental functions and are often implicated in the etiology of placental pathologies. Recent advancements in cell biology have facilitated the development of innovative in vitro STB model systems. However, as the variety of available in vitro STB models grows, a critical assessment of the strengths, limitations, and appropriate applications of both established and emerging model systems is important for the field. OBJECTIVE AND RATIONALE:With this review, we set out to compile and synthesize current knowledge on in vitro modeling of STB. Using this information, we sought to develop a balanced and thoughtful discussion regarding the use and suitability of various in vitro STB models. Our approach is grounded in a framework that considers placental development and physiology, with a specific focus on the capability of different models to recapitulate and thus enable the study of human STB differentiation, development, function, and dysfunction. SEARCH METHOD:This review assessed published literature sourced through the PubMed database. Search terms included 'human placenta models,' 'syncytiotrophoblast models,' 'syncytiotrophoblast development,' 'trophoblast stem cells,' 'trophoblast organoids,' and 'trophoblast cell models.' The literature search was limited to English-language publications available up to August 2025. OUTCOMES:We provide a narrative which explores the features, potential applications, and limitations of various STB models, including explant systems, immortalized trophoblast cell lines, stem cell-derived trophoblast, and a range of established and emerging 3D culture systems. Our evaluation focuses on the potential of each model to address specific research questions and highlights the challenges associated with modeling different stages of STB development and different unique aspects of STB functionality. Moreover, while remarkable progress in developing STB models has been made, no single system fully recapitulates the complex in vivo features of STB formation and function. Rather than being exhaustive, this review seeks to provide an evidence-based perspective on STB modeling in vitro which can encourage the careful consideration of the strengths and limitations of STB models. WIDER IMPLICATIONS:This review provides an overview of the in vitro STB models currently available and a commentary of the knowledge that these systems have contributed to our understanding of STB biology. While the field has made significant progress, ongoing refinement of existing models is essential for advancing our understanding of STB and their role in both the health and dysfunction of the human placenta. By summarizing the unique adaptations and physiological changes of STB throughout gestation and aligning these with the capabilities of current models, we have developed a framework to guide future research and innovation in STB modeling. This framework is underscored by the importance of selecting models which align with specific research questions and simultaneously acknowledging the inherent limitations in extrapolating data from any in vitro systems to the biological context of the developing human placenta. By generating this discussion, we hope to contribute to the ongoing refinement of placental research methodologies and to inspire continued innovation in STB model systems. REGISTRATION NUMBER:N/A.
BACKGROUND:Polycystic ovary syndrome (PCOS) affects women globally, but its prevalence across World Health Organization (WHO) regions has not previously been reported. OBJECTIVE AND RATIONALE:We aimed to synthesize evidence on the prevalence of PCOS by diagnostic criteria and by WHO geographic regions to inform the International Evidence-Based PCOS Guideline. SEARCH METHODS:A systematic search of OVID MEDLINE, All EBM, PsycInfo, EMBASE, and Cumulative Index to Nursing and Allied Health Literature was conducted from 1990 to November 2024. Studies assessing PCOS prevalence in an unselected population were included. Non-primary studies or those with unclear diagnostic criteria were excluded. The primary outcome was PCOS prevalence among adult women. The secondary outcome was PCOS prevalence among women of all ages. Random effects meta-analysis using the DerSimonian and Laird method was applied for estimating the overall effect size. Two reviewers independently assessed risk-of-bias (RoB) and evidence certainty. OUTCOMES:The search yielded 16 664 articles, of which 119 unique studies (in 137 articles) were eligible, and 92 (including 157 181 participants) were pooled in a meta-analysis. By diagnostic criteria, PCOS global prevalence among adult women only was 12.1% (95% CI: 9.8, 14.8; I2: 98.8%) using Rotterdam criteria, 7.9% (95% CI: 6.2, 9.9; I2: 96.2%) using the original National Institute of Health (NIH) criteria, 12.7% (95% CI: 8.2, 17.9; I2: 98.0%) using the Androgen Excess (AE)-PCOS criteria, and 7.8% (95% CI: 5.8, 10.0; I2: 99.4%) by self-report. By WHO regions, PCOS prevalence among adult women when using Rotterdam criteria was highest in the Eastern Mediterranean region (15.1%; 95% CI: 11.1, 19.7) and the South-East Asian region (14.3%; 95% CI: 5.8, 25.9), followed by the European region (11.7%; 95% CI: 5.1, 20.3), the region of the Americas (10.5%; 95% CI: 3.0, 21.7), and the Western Pacific region (9.1%; 95% CI: 6.2, 12.5), with no data from Africa. Subgroup analysis using Cochran's Q test indicated a statistically significant difference in prevalence by WHO region (P = 0.022). Subgroup analyses including adolescents yielded a lower prevalence globally, with a global prevalence of 11.4% (95% CI: 9.5, 13.5) by Rotterdam criteria, 7.1% (95% CI: 5.7, 8.7) by NIH criteria, 11.2% (95% CI: 7.4, 15.5) by AE-PCOS criteria, and 7.6% (95% CI: 5.8, 9.6) on self-report. Of the 119 studies, 30 had low, 49 had moderate, and 40 had high RoB. Certainty of evidence ranged from very low to low. WIDER IMPLICATIONS:This is the most comprehensive and contemporary review of PCOS prevalence and highlights past inconsistencies in diagnostic criteria and individual diagnostic features. Pooled PCOS prevalence was 12.1% by the Rotterdam criteria and was highest in the Eastern Mediterranean and the South-East Asian regions, with a potentially different health burden of PCOS across world regions. These findings directly inform International PCOS Guidelines, including updated guideline diagnostic criteria and refined individual features, emphasizing early, accurate diagnosis. REGISTRATION NUMBER:PROSPERO CRD42022372029.
BACKGROUND:Stem cell-based embryo models (SCBEMs) are clusters of pluripotent stem cells that can mimic morphological and functional aspects of early human embryos to different degrees. When cultured from human cells, SCBEMs offer technically scalable and amenable tools that can help refine, reduce, and, in the future, perhaps replace the use of animals and human embryos in fundamental and clinical research. These advantages propelled the development of SCBEMs, and several distinct types have been generated over the past decade, including gastruloids, axioloids, blastoids, and post-implantation-like embryoids. For purposes of governance, advisory reports distinguish between SCBEMs based on their presumed capacity to continuously undergo organized human development-referred to here as developmental potential. However, since functionally testing this potential by transferring human SCBEMs to a uterus would be unethical and is recommended to be prohibited, scientists lack clear or consistent ways to assess it. OBJECTIVE AND RATIONALE:This narrative review aims to tackle the question of how to assess developmental potential in SCBEMs by clarifying the different ways in which it can be and is being conceptualized. We achieve this by synthesizing insights from governance, science, and ethics. First, we examine how developmental potential is described in contemporary governance frameworks, and which aspects are emphasized. Next, we discuss biological markers for developmental potential and show how their scientific basis (in embryos, let alone SCBEMs) remains poorly understood. Then, we explore how the aspects considered relevant for assessments of developmental potential in governance and science may pre-emptively hinge on underlying conceptual interpretations and lead to differing normative implications. SEARCH METHODS:This narrative review combines insights from both the academic and grey literature on the (ethics of) embryo models. Original and review articles were selected from PubMed and Biorxiv with the main focus on articles published since 2015. Search terms included: embryo quality, in vitro fertilization, Gardner system, blastoid, gastruloid, embryo research, potentiality argument, developmental potential, transcriptomics, epigenetics, embryo metabolism, and related terms. Additional sources were identified through snowballing. This work focuses predominantly on human SCBEMs, but references to animal models are made. OUTCOMES:Comparison of the descriptions currently recommended for governance suggests at least three criteria that are used to assess developmental potential in SCBEMs: composition, organization, and interaction. Scientifically, developmental potential is multifaceted and only partly characterized, making it necessary to measure a broader range of aspects, using human embryos as benchmarks when possible. Since the range and significance of these aspects can be shaped by underlying accounts of developmental potential, contemporary advisory reports are examined to explore if and how they connote interpretations of developmental potential as possibility, probability, and predisposition. WIDER IMPLICATIONS:Categorization of the regulatory and scientific criteria currently used to assess developmental potential shows that they are underpinned by distinct interpretations of the concept, revealing tensions and questions for further inquiry. By synthesizing insights from governance, science, and ethics, this review thus aims to contribute to the responsible advancement of the SCBEM field and to support its coherent and transparent governance. REGISTRATION NUMBER:N/A.
BACKGROUND:Mammalian embryo development involves a complex process governed by multiple layers of cellular and molecular regulation mechanisms. ART is widely used around the world to assist fertility in humans, with ∼12 million babies being born by ART in the last 40 years. These technologies are also used extensively for reproductive purposes in other mammalian species that have many analogies with human reproductive biology. Epitranscriptomic marks, including RNA modifications such as N6-methyladenosine (m6A) and N1-methyladenosine (m1A), modulate gene expression during gametogenesis and embryo development, and their dynamics are regulated by genes encoding m6A writers (METTL3, METTL14, and WTAP), readers (YTHDF2, YTHDC1-2, and PRRC2A), and erasers (ALKBH5 and FTO). However, the impact of ART on these epigenetic modifications remains poorly understood. OBJECTIVE AND RATIONALE:This narrative review explores the role of epitranscriptomic modifications in both naturally and ART-conceived embryos. It examines how RNA modifications regulate gametogenesis and early embryonic development and how ART-induced cellular stress might perturb these regulatory layers, potentially affecting gametogenesis, embryo competence, and offspring health. Understanding the interaction between ART and epitranscriptomic regulation is crucial for optimizing ART procedures and safeguarding offspring health. SEARCH METHODS:The PubMed and Scopus literature databases were utilized to search for peer-reviewed articles and reviews using terms such as 'epitranscriptomic', 'RNA modification', 'gametogenesis', 'embryo development', 'mammalian development', 'in vitro fertilization', 'ART', and 'assisted reproductive technologies' in combination or individually. All relevant publications until the current year have been critically evaluated and discussed. OUTCOMES:Epitranscriptomic modifications, particularly m6A, have emerged as key regulators of RNA metabolism during gametogenesis and early embryo development. Evidence from both human and animal studies indicates that ART-related stressors, such as oxidative imbalance, hormonal stimulation, and cryopreservation, can disturb RNA methylation at the epitranscriptomic marks m1A and 5-methylcytosine by modulating the expression and activity of m6A writers, erasers, and readers, independently of global transcriptional changes. These alterations can affect embryo competence, placental function, lineage specification, and subsequent offspring development. Moreover, m6A-associated factors participate in stress adaptation and developmental signalling beyond their canonical methylation activity. Collectively, these findings underscore the remarkable sensitivity of the embryonic transcriptome to in vitro manipulation and highlight epitranscriptomic marks as both predictive biomarkers and mechanistic targets for improving the safety, efficacy, and long-term outcomes of assisted reproduction. WIDER IMPLICATIONS:Understanding how ARTs influence the epitranscriptome and its downstream effects is crucial for improving reproductive outcomes. In vitro manipulation, fertilization, and embryo culture can influence RNA regulation in gametes, causing reduced cell differentiation, and, in early embryos, contributing to recurrent implantation failure, decidualization failure, and pregnancy loss. This review aims to share with the scientific community insights into the critical role of epitranscriptomic modifications during gametogenesis and embryogenesis, as well as the potential consequences of in vitro procedures, to guide safer and more effective ART practices. REGISTRATION NUMBER:N/A.
