Birth of Thirty-Two Healthy Babies Following Transfer of Fresh and Frozen–Thawed Embryos Derived from Monopronuclear Zygotes: A Retrospective Study

Abstract

Background and Objectives: Fertilized zygotes normally display two pronuclei (PN), but abnormal fertilization patterns (0, 1 or >2PN) are observed daily in IVF labs. Multiple PN zygotes (>2) are generally discarded due to an increased risk of aneuploidy. However, the decision to transfer or not transfer 1PN-derived embryos remains controversial. The aims of our study were to analyze the neonatal outcomes of fresh or frozen–thawed embryos derived from 1PN zygotes, and to evaluate the influence of the fertilization method. Materials and Methods: Data were retrospectively collected from cycles performed between January 2018 and December 2022. Fresh cycles were analyzed for the comparative fate of 1PN zygotes (n = 1234) following conventional in vitro fertilization (cIVF; n = 648) or intracytoplasmic sperm injection (ICSI; n = 586), as well as the results of the 64 transfers of 1PN-derived embryos (pregnancy rate (PR) and neonatal outcomes). This pregnancy follow-up was also applied to 167 transfers of frozen–thawed 1PN-derived embryos. Results: In fresh cycles, 46% of the 1PN zygotes in the cIVF group developed into embryos of sufficient quality to be transferred or frozen (day 3 or 5/6). This rate was lower in the fresh ICSI cycles (33%). Blastulation rate was also significantly higher in the cIVF group (44%) in comparison to the ICSI group (20%). The fresh single embryo transfers (32 per group) allowed seven pregnancies in the cIVF group (PR = 21.9%) as compared to four pregnancies in the ICSI group (PR = 12.5%). In the cIVF group, five deliveries of healthy newborns were achieved, but only one in the ICSI group. In frozen/thawed cycles, 36 pregnancies were obtained out of the 167 transfers. A non-significant difference was observed between embryos derived from cIVF cycles (PR = 26%) and ICSI cycles (PR = 16%) with 18 and 8 healthy babies born, respectively. Conclusions: We observed better outcomes for 1PN zygotes in cIVF cycles in comparison to ICSI cycles. Our center policy to transfer good-quality 1PN-derived embryos allowed the birth of 32 healthy babies.