Impact of no zona pellucida on reproductive outcomes in vitrified-warmed euploid blastocyst transfer cycles.

OBJECTIVE To evaluate the impact of the absence of the zona pellucida on clinical, obstetric, and perinatal outcomes in vitrified-warmed euploid blastocyst transfer cycles. DESIGN Retrospective cohort study. SUBJECTS This retrospective study included data from 1,024 preimplantation genetic testing-frozen embryo transfer cycles, conducted at a tertiary academic medical center between March 2016 and December 2021. EXPOSURE Vitrified-warmed single euploid blastocyst transfers were classified into zona pellucida-intact (unexposed; n=761) and zona pellucida-free (exposed; n=263) groups. Frozen embryo transfer cycles in the zona pellucida-free group were matched with those in the zona pellucida-intact group at a 1:1 ratio using propensity score matching. MAIN OUTCOME MEASURES The primary outcome was live birth. Secondary outcomes were clinical pregnancy, miscarriage, obstetric outcomes including cesarean delivery and gestational age, as well as perinatal outcomes including birth weight and birth defects. RESULTS Before propensity score matching, the zona pellucida-free group exhibited significantly lower rates of clinical pregnancy (51.71% vs. 63.73%, P=0.001) and live birth (44.11% vs. 56.64%, P<0.001), as well as a higher rate of cesarean delivery (81.03% vs. 70.30%, P=0.018). However, these differences were no longer statistically significant (clinical pregnancy, 53.52% vs. 54.46%, P>0.05; live births, 45.54% vs. 49.77%, P>0.05; cesarean delivery, 80.41% vs. 74.53%, P>0.05) after matching. Multivariate analysis revealed that the embryo development stage (day 6 vs. day 5: odds ratio=0.70, P=0.029) and blastocyst quality (poor vs. good: odds ratio=0.47, P<0.001) were significant predictors of live birth. Furthermore, maternal age (odds ratio=1.09, P=0.003) and endometrial thickness (odds ratio=0.83, P=0.015) were significantly associated with the likelihood of a cesarean delivery. CONCLUSION The absence of the zona pellucida did not independently affect clinical, obstetric, or perinatal outcomes in vitrified-warmed single euploid blastocyst transfers. Criteria for embryo selection, particularly developmental stage and quality, were critical determinants of successful pregnancy outcomes.