Maternal age at frozen embryo transfer, not cryopreservation duration, is more important in the transfer of cleavage-stage embryos than blastocysts: A single-center retrospective study.

This study aims to explore the effect of various factors, including freezing duration, on frozen embryo transfer outcomes. We analyzed 2291 frozen-thawed cycles based on embryo freezing time (12 months). There were no significant differences in the rates of clinical pregnancy, live birth, miscarriage, or preterm labor between the groups. The intergroup difference in the birth weight of singletons was significant (P = .012). Following cleavage-stage embryo transfer, maternal age at frozen embryo transfer (odds ratio OR, 95% confidence interval CI = 0.813 0.674-0.981, P = .030), (OR 95% CI = 0.779 0.635-0.955, P = .016), and number of embryos transferred (OR 95% CI = 1.527 1.172-1.989, P = .002), (OR 95% CI = 1.688 1.270-2.242, P < .001) were associated with the clinical pregnancy and live birth rates, respectively. Following blastocyst-stage embryo transfer, cycle number of transplantation (OR 95% CI = 0.570 0.369-0.881, P = .011), (OR 95% CI = 0.565 0.366-0.871, P = .010), and number of embryos transferred (OR 95% CI = 1.734 1.296-2.322, P < .001), (OR 95% CI = 1.951 1.460-2.606, P < .001) were associated with the clinical pregnancy and live birth rates, respectively. Ovarian stimulation protocol (OR 95% CI = 1.511 1.006-2.268, P = .047) was associated only with the clinical pregnancy rate. Embryo cryopreservation duration did not affect the clinical pregnancy or live birth rates regardless of embryo stage. Older patients could consider blastocyst embryo culture and transplantation to improve clinical pregnancy and live birth rates.