Timing of frozen-thawed embryo transfer after controlled ovarian stimulation in a non-elective freeze-all policy

Background: Non-elective freeze-all policy has been increasingly utilized in assisted reproductive treatment, but the optimal timing of frozen-thawed embryo transfer (FET) after controlled ovarian stimulation (COS) remains to be investigated. Methods: This retrospective cohort study included 2,998 patients who underwent their first FETs after the first COS cycles using the non-elective freeze-all strategy from Jan 2013 to Dec 2016 at a tertiary-care academic medical center. Patients were divided into the "immediate" group in which FET took place within the first menstrual cycle after oocyte retrieval, and the "delayed" group where FET started after one or more menstrual cycles following COS. Results: The mean interval between oocyte retrieval and FET was 33.3 +/- 5.8 days in the immediate group (n=280; 9.3%) and 91.3 +/- 19.4 days in the delayed group (n=2,718; 90.7%). Cycles with delayed FET had a significantly lower live birth rate than those with immediate FET before [1,246/2,718 (45.8%) vs. 156/280 (55.7%); P=0.002] and after propensity score matching (PSM) [123/280 (43.9%) vs. 156/280 (55.7%); P=0.005]. When controlling for a number of confounding factors by multivariable logistic regression analysis, the risk remained significant with the adjusted odds ratio (aOR) [95% confidence interval (CI)] of 0.69 (0.53-0.90) and 0.60 (0.42-0.85) before and after matching, respectively. Conclusions: Performing FET immediately within the first menstrual cycle following COS was associated with a higher chance to achieve live birth compared with delaying FET to subsequent cycles in a non-elective freeze-all policy. However, further randomized controlled trials are still needed to confirm this conclusion.