[The effect of the time interval between oocyte retrieval in the fresh cycle and the first frozen embryo transfer on pregnancy outcomes].

Objective: To investigate the effect of the time interval between full embryo freezing in the fresh cycle and the first frozen embryo transfer on pregnancy outcomes. Methods: A retrospective cohort study was conducted. Patients who underwent full embryo freezing after assisted reproduction with in vitro fertilization/intracytoplasmic sperm injection using the follicular-phase long-acting protocol in the fresh cycle at the Reproductive Medicine Center of the First Affiliated Hospital of Zhengzhou University from January 2019 to December 2021 were included, with a total of 2 408 cases. Patients were divided into three groups according to the interval between oocyte retrieval and frozen embryo transfer: ≤60 d (n=573), 60-90 d (n=1 303), and>90 d (n=532). Restricted cubic spline modeling and multivariable logistic regression were used to analyze the effects of different oocyte retrieval-to-frozen embryo transfer intervals on pregnancy outcomes. Results: Patients were at a mean age of (29.5±3.9) years, with mean ages of (29.3±3.8), (29.3±3.9), and (30.1±3.9) years in the ≤60 d, 60-90 d, and >90 d groups, respectively. The ≤60 d group had lower clinical pregnancy rates [49.2% (282/573) vs 56.8% (740/1 303) vs 60.9% (324/532)] and live-birth rates [71.6% (202/573) vs 80.8% (598/1 303) vs 79.0% (256/532)] compared with the 60-90 d and >90 d groups (all P<0.05). No significant differences were observed among the groups in terms of obstetric complications, neonatal birth weight, or birth defects (all P>0.05). Oocyte retrieval-frozen embryo transfer interval in<60 d group (aOR=0.59, 95%CI: 0.46-0.75; aOR=0.56, 95%CI: 0.43-0.72), age (aOR=0.98, 95%CI: 0.96-0.99; aOR=0.97, 95%CI: 0.95-0.99), natural-cycle endometrial preparation protocol (aOR=1.41, 95%CI: 1.15-1.74; aOR=1.37, 95%CI: 1.12-1.68), endometrial thickness on the day of transfer (aOR=1.10, 95%CI: 1.00-1.22; aOR=1.11, 95%CI: 1.01-1.22), number of embryos transferred (aOR=2.79, 95%CI: 1.50-5.17; aOR=3.12, 95%CI: 1.63-5.97), and D3 embryo type (aOR=0.42, 95%CI: 0.23-0.78; aOR=0.42, 95%CI: 0.22-0.81) were all factors associated with successful clinical pregnancy or live birth (all P<0.05). None of the variables in the 61-90 and >90 d groups were significantly associated with successful clinical pregnancy or live birth (all P>0.05). A nonlinear association was identified between the oocyte retrieval-to-frozen embryo transfer interval and both clinical pregnancy (Poverall<0.001, Pnon-linear<0.001) and live-birth rates (Poverall<0.001, Pnon-linear=0.005), characterized by an initial rise followed by a plateau, with thresholds at 65 and 64 days, respectively. Below the thresholds, each additional day of interval increased the clinical pregnancy rate by 6.2% (aOR=1.06, 95%CI: 1.04-1.09) and the live-birth rate by 5.6% (aOR=1.06, 95%CI: 1.03-1.09) (both P<0.05). Conclusions: The timing of frozen embryo transfer after whole-embryo cryopreservation in fresh cycles influences pregnancy outcomes. Performing frozen embryo transfer approximately two months after oocyte retrieval is associated with improved pregnancy rate.