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 <i>vs.</i> 88.7 ± 35.5 d, <i>P</i> < 0.001). Regarding pregnancy results, the immediate cycles resulted in comparable implantation rate (55.3% <i>vs</i>. 57.2%, <i>P =</i> 0.624), clinical pregnancy rate (66.9% <i>vs</i>. 72.2%, <i>P =</i> 0.236), abortion rate (16.1% <i>vs</i>. 12.7%, <i>P =</i> 0.402), ongoing pregnancy rate (59.0% <i>vs</i>. 65.0%, <i>P =</i> 0.204), and live birth rate (56.1% <i>vs</i>. 63.0%, <i>P =</i> 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 <i>vs</i>. 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.