Association between embryo cryopreservation duration and pregnancy outcomes in women receiving the freeze-all strategy: A multicentered retrospective study.

Abstract

Objective: To investigate the impact of embryo cryopreservation duration on the clinical pregnancy rate (CPR) and live birth rate (LBR) in women undergoing their first frozen embryo transfer (FET) following the freeze-all strategy.

Methods: This multicenter retrospective study included 32 838 women who were patients at the five large-scale assisted reproduction treatment (ART) centers in China. The subjects were divided into seven groups according to the duration of embryo cryopreservation, among which the CPRs and LBRs were compared. Multivariate logistic regression and restricted cubic spline (RCS) analysis were used to assess the association between the duration of cryopreservation and the CPR/LBR outcomes. Furthermore, we performed subgroup analyses within the cohort of women who underwent FET within 3 months to investigating whether the impact cryopreservation duration on clinical outcomes varied under different estradiol (E₂) level on the trigger day of the ovarian stimulation cycles (E₂ ≤ 1000 pg/mL, or E₂ > 1000 pg/mL).

Results: The CPR and LBR increased to the highest values (64.2% and 54.5%, respectively) during the first early 3 months of cryopreservation, and subsequently decreased as the cryopreservation duration increased, exhibiting an inverted U-shaped pattern. Further subgroup analyses within the first 3 months revealed that the cryopreservation duration did not significantly affect the CPR and LBR in the group with E₂ ≤ 1000 pg/mL, whereas in the subgroup with E₂ > 1000 pg/mL, the CPR and LBR increased with the prolonged cryopreservation.

Conclusion: Our results suggest that a 3-month cryopreservation duration is associated with better FET outcomes, especially for patients with E₂ > 1000 pg/mL. Moreover, performing FET too early or too long after embryo cryopreservation may adversely affect clinical outcomes.