The value of the pronucleus size of human single pronucleus zygotes as a predictor of fertility outcomes in conventional in vitro fertilization: a retrospective cohort study

BACKGROUND: Embryos derived from in vitro fertilization (IVF) single pronucleus (1PN) zygotes can achieve live births. The pronucleus size of 1PN zygotes is related to their embryonic development potential. However, these studies cannot draw clear conclusions because of the limited sample size. Currently, methods for accurately predicting the development potential of 1PN blastocysts on the basis of the pronucleus diameter are lacking. In this study, we aimed to assess the effects of the pronucleus size of human 1PN zygotes derived from conventional IVF on embryological and pregnancy outcomes. METHODS: This retrospective cohort study included 629 IVF-1PN cycles from 514 individuals between May 2023 and April 2024 to observe embryonic development. Pregnancy outcomes were assessed in 36 patients who received their first transfer of single vitrified-warmed blastocysts derived from IVF-1PN zygotes. Human IVF-1PN zygotes were classified into six groups by PN diameter (≤ 20, 25, 30, 35, 40, and ≥ 45 μm), with the cutoff value determined by examining the receiver operating characteristic curve. RESULTS: Compared with 1PN zygotes with smaller PN diameters, 1PN zygotes with larger PN diameters had significantly greater cleavage rates, blastocyst formation rates, and blastocyst qualities (P < 0.05). The diameter of 1PN zygotes is a valuable indicator for predicting blastocyst formation (AUC = 0.605, cutoff = 32.5 μm). Moreover, the transfer of IVF-1PN blastocysts with different PN diameters resulted in similar pregnancy outcomes (biochemical pregnancy, ongoing pregnancy, and live birth rates). CONCLUSIONS: Our results suggest that 1PN zygotes derived from conventional IVF with larger PN diameters have better embryonic development and could be used as predictors of blastocyst formation (> 32.5 μm). However, once the zygotes reach the blastocyst stage, the PN diameter is not associated with pregnancy outcomes.