Objective: To explore how seasonal variation in maternal vitamin D levels may contribute to adverse neonatal outcomes, given rising concerns over widespread deficiency and its impact on maternal-fetal health. Design: Systematic review of observational studies using seasonal maternal vitamin D measurements and reported neonatal outcomes, including anthropometric values (birth weight, length, head circumference) and preterm birth. Materials and Methods: A comprehensive literature search was conducted across four databases: PubMed, CINAHL Plus, Cochrane Library, and SCOPUS; combinations of keywords and/or MeSH terms guided the search, which yielded 1,853 articles. After removal of duplicates and abstract screening using predefined criteria – including pregnant women, maternal vitamin D level, season, and neonatal outcomes, and excluding vitamin D supplementation, non-English studies, and systematic reviews – 39 articles were selected for full-text review. Articles were screened in Rayyan by multiple reviewers, and data were manually extracted. Results: Following full-text review, 11 studies met the inclusion criteria. Eight studies confirmed significant seasonal variation in maternal vitamin D levels, with a high prevalence of deficiency, particularly in winter months. Five of the articles described statistically significant associations between maternal vitamin D levels and neonatal outcomes. Among these, preterm birth showed the most consistent correlations, two studies linked vitamin D deficiency to low birth weight, but none found associations for birth length or head circumference. Conclusions: Vitamin D deficiency among pregnant women, especially in winter months and areas with limited sun exposure, warrants greater clinical and public health attention. Large-scale longitudinal studies are needed to clarify the role of vitamin D in preventing adverse pregnancy outcomes, and to inform evidence-based recommendations.
