Comparative analysis of pregnancy outcomes in different time intervals following GnRH antagonist stimulation and modified natural frozen-thawed embryo transfers.

To evaluate whether the interval between oocyte retrieval and frozen-thawed embryo transfer (FET) affects clinical outcomes using modified natural cycle (mNC) regimen following freeze-all GnRH antagonist stimulation. A total of 542 patients underwent first mNC-FET were divided into two groups: immediate (FET conducted in the first menstrual cycle after freeze-all) and delayed (FET performed in the second or subsequent menstrual cycle) groups. The immediate group was associated with a notably reduced duration between oocyte retrieval and transfer day (35.5 ± 5.8 vs. 88.7 ± 35.5 d, P < 0.001). Regarding pregnancy results, the immediate cycles resulted in comparable implantation rate (55.3% vs. 57.2%, P = 0.624), clinical pregnancy rate (66.9% vs. 72.2%, P = 0.236), abortion rate (16.1% vs. 12.7%, P = 0.402), ongoing pregnancy rate (59.0% vs. 65.0%, P = 0.204), and live birth rate (56.1% vs. 63.0%, P = 0.149) with the delayed cycles. After the multivariable logistic regression analysis, the difference in live birth outcome between the two regimens remained insignificant (immediate vs. delayed protocol: adjusted odds ratio = 0.87, 95% confidence interval= 0.56-1.33). Immediate mNC-FET following a freeze-all antagonist cycle displayed promising clinical outcomes and significantly shortened the time to conception compared to delayed FET.