Evaluating the developmental potential of 2.1PN-derived embryos and associated chromosomal analysis

Zygotes with 2.1 pronuclei (2.1PN) present with two normal-sized pronuclei, and an additional smaller pronucleus, that is approximately smaller than two thirds the size of a normal pronucleus. It remains unclear whether the additional pronucleus causes embryonic chromosome abnormalities. In the majority of cases, in vitro fertilization (IVF) clinics discarded 2.1PN zygotes. Thus, the present study aimed to evaluate the developmental potential and value of 2.1PN zygotes. 2.1PN-derived embryos from 164 patients who underwent IVF or intracytoplasmic sperm injection (ICSI) treatment between January 2021 and December 2022 were included in the present study. All embryos were monitored using a time-lapse system, and blastocyst formation was used to assess 2.1PN-derived embryo developmental potential. The blastocyst formation was quantified using generalized estimating equations, and chromosome euploidy was analyzed using next-generation sequencing (NGS). In addition, the potential association between age and occurrence of 2.1PN zygotes was determined. The present study demonstrated that numerous 2.1PN zygotes developed into blastocysts. Early cleavage patterns and embryo quality on Day 3 were the independent predictors for the blastocyst formation of 2.1PN-derived embryos. The 2.1PN zygotes displayed a comparable developmental potential compared to 2PN zygotes in advanced age patients (≥ 38). Moreover, there was a tendency that 2.1PN-derived blastocysts showed a similar euploidy rate compared to 2PN-derived blastocysts. Clinicians should consider using 2.1PN-derived euploid embryos for transfer after preimplantation genetic testing in the absence of available 2PN embryo cycles. 2.1PN-derived embryos could be a candidate, particularly beneficial for patients at advanced age.