Association between endometrial echo on transfer day and pregnancy outcomes in thawed embryo transfer: a retrospective cohort study across different preparation protocols

Objective This study aimed to investigate the relationship between endometrial echo and pregnancy outcome in patients undergoing thawed embryo transfer and explore the effect of different endometrial preparation schemes on endometrial echo. Methods A retrospective analysis was conducted on data from 2910 patients who underwent freeze-thaw embryo transfer (FET)-assisted pregnancy in the reproductive medicine centre of our hospital from January 2019 to March 2024. Based on the endometrial echo on the transplantation day, the patients were divided into two groups: the endometrial echo uniform group and the endometrial echo uneven group. Based on the endometrial preparation protocol, they were divided into the natural cycle (NC) group, hormone replacement cycle (HRT) group, and downregulated combined HRT (GnRHa+HRT) group. The general data, pregnancy outcome and endometrial echo of those undergoing different endometrial preparation protocols were compared. Results The clinical pregnancy rate in the homogeneous endometrial echo group was significantly higher than that in the non-homogeneous endometrial echo group. The equalisation rate of endometrial echo in the NC group and GnRHa+HRT group was significantly higher than that in the HRT group (p<0.05). Binary logistic regression analysis revealed that homogeneous endometrial echo was associated with a significantly improved clinical pregnancy rate of patients with FET. After adjusting for confounding factors, we found that compared with the HRT endometrial preparation protocol, NC and GnRHa+HRT endometrial preparation protocol significantly improved homogeneous endometrial echo. Conclusion Non-homogeneous endometrial echo on the transplantation day was associated with a decreased clinical pregnancy rate of FET. The endometrial preparation protocol of the NC and GnRHa+HRT cycle can improve homogeneous endometrial echo. In FET, clinicians should develop a personalised endometrial preparation protocol based on patients’ situations.