Cryopreservation duration does not affect pregnancy or neonatal outcomes in single high-quality blastocyst transfers: a multicenter retrospective study

Objective: To study the effect of embryo cryopreservation duration on pregnancy and neonatal outcomes in women transferred with high-quality blastocyst during frozen embryo transfer (FET) cycles. Design: Multicenter Retrospective cohort study. Setting: Three tertiary-care academic medical centers. Patients: This retrospective study included a total of 24,101 women who underwent single high-quality blastocyst transfer during their first FET cycles at three tertiary academic medical centers between January 2016 and June 2023. Interventions: Women were categorized into two groups according to the duration of embryo cryopreservation: the short Cryo group consisted of 23,933 women with a storage time of 0-5 years, while the long Cryo group included 168 women with a storage time > 5 years. Women in the long Cryo group were matched to those in the short Cryo group using propensity score matching with a 1:4 ratio. Main outcome measures: The pregnancy outcomes and the neonatal outcomes. Results: After adjusting for potential confounding factors, no significant differences were observed between the two groups in pregnancy outcomes, including biochemical pregnancy (adjust odds ratio [aOR] 1.04, 95% confidence interval [CI], 0.70-1.56; P = 0.831), clinical pregnancy (aOR 1.10, 95% CI, 0.75-1.60; P = 0.638), ectopic pregnancy (aOR 2.15, 95% CI, 0.37-12.56; P = 0.394), miscarriage (aOR 0.95, 95% CI, 0.54-1.69; P = 0.871), and live birth (aOR 1.09, 95% CI, 0.76-1.55; P = 0.646). In addition, no significant differences were observed in neonatal outcomes, including very preterm birth, preterm birth, very low birth weight, low birth weight, high birth weight, birth weight, and gestational age. Conclusions: Our analysis found no evidence of significant associations between prolonged cryopreservation of high-quality blastocysts and adverse pregnancy or neonatal outcomes.