Study on the relationship between endometrial thickness on hCG day in the progestin-primed ovarian stimulation protocols and pregnancy outcomes in frozen-thawed embryo transfer cycles

Yutong Wang^1,2,3Yajie Chang^1,2,3Shaohong Zhuang^1,2,3Haitao Zeng^1,2,3Xiaoyan Liang^1,2,3*

• ¹Reproductive Medicine Center, Sixth Affiliated Hospital of Sun Yat-sen University, Guangzhou, China
• ²Guangdong Engineering Technology Research Center of Fertility Preservation, Guangzhou, China
• ³Biomedical Innovation Center, The Sixth Affiliated Hospital of Sun Yat-sen University, Guangzhou, China

Objective To investigate the relationship between endometrial thickness (EMT) on hCG day in the progestin-primed ovarian stimulation (PPOS) protocol and pregnancy outcomes in the following frozen-thawed embryo transfer (FET) cycles, providing new insights into embryo transfer strategies for patients undergoing PPOS protocols. Methods A retrospective study including 1,748 patients underwent PPOS protocols were included. Patients were divided into two groups based on EMT on hCG day: Group A (<8 mm) and Group B (≥8 mm). After 1:1 propensity score matching (Group A: n=701, Group B: n=701), biochemical pregnancy rates, clinical pregnancy rates, and ongoing pregnancy rates were compared. Additionally, pregnancy outcomes under different endometrial preparation protocols were analyzed within each group. Results 1. The biochemical pregnancy rates in Groups A and B were 44.5% and 45.5% (p=0.707), the clinical pregnancy rates were 38.5% and 42.5% (p=0.128), and the ongoing pregnancy rates were 29.1% and 34.2% (p=0.039). For cleavage-stage embryo transfers, Groups A and B showed no significant differences in biochemical pregnancy rates (39.5% vs. 35%, p=0.285), clinical pregnancy rates (35.9% vs. 30.5%, p=0.192), and ongoing pregnancy rates (26.8% vs. 24.4%, p=0.527). For blastocyst transfers, biochemical pregnancy rates (47.8% vs. 51.2%, p=0.307), clinical pregnancy rates (40.2% vs. 49%, p=0.009), and ongoing pregnancy rates (30.6% vs. 39.6%, p=0.005) were significantly higher in Group B; 2. In Group A, the endometrial preparation protocols had no statistically significant effect on pregnancy outcomes, but the natural cycles showed potentially better results (biochemical pregnancy rate: 43.1%, clinical pregnancy rate: 40.0%, ongoing pregnancy rate: 34.1%) than HRT (44.1%, 38.6%, 29.9%) , GnRH-a+HRT (45.1%, 39.3%, 26.3%), and mild stimulation cycles (27.8%, 16.7%, 11.1%). In Group B, patients using GnRH-a+HRT protocols showed higher biochemical pregnancy rates (53.5%, p=0.022), clinical pregnancy rates (48.7%, p=0.096), and ongoing pregnancy rates (40.4%, p=0.032) compared to the natural cycles (38.2%, 33.9%, 25.2%), HRT (42.9%, 40.8%, 33.8%), and mild stimulation protocols (45.5%, 45.5%, 27.3%). Conclusions EMT on the hCG day in PPOS cycles was positively related to pregnancy outcomes of subsequent FET, with EMT ≥8 mm associated with better pregnancy outcomes, especially in patients undergoing blastocyst transfers.