Impact of endometrial thickness before frozen-warmed blastocyst transfer on live birth rate in women with endometriosis

The effect of endometrial thickness on live birth rates after embryo transfer, and whether there is a threshold below which the chances of live birth decrease, remains unclear. Although the eutopic endometrium in women with endometriosis exhibits various molecular and cellular alterations across multiple pathways, whether endometrial receptivity is impaired in these patients remains controversial. The aim of our study is to evaluate the role of endometrial thickness on live birth rates in single frozen embryo transfer cycles, comparing women with and without endometriosis. This retrospective cohort study includes autologous single blastocyst transfers from frozen-thawed cycles in women with and without endometriosis who underwent embryo transfer between January 2017 and June 2023 at the University Hospital Zurich, Switzerland. All transfers were performed using artificial endometrial preparation for a single blastocyst transfer. Out of 640 autologous single frozen embryo transfers, we included 95 single blastocyst transfers in women with endometriosis and 190 in women without endometriosis after propensity score matching. Conditional density plots for both groups did neither demonstrate a linear relationship between the endometrial thickness on the day of progesterone administration and clinical pregnancy rate (CPR) or live birth rate (LBR), nor did they reveal a clear threshold below which CPR or LBR decreased noticeably. The peak LBR in women without endometriosis was observed at an endometrial thickness of 8 mm, whereas in women with endometriosis, the peak was at 10 mm. Receiver operating characteristic (ROC) analyses did not suggest a predictive value of endometrial thickness for live birth in either group, with area under the curve (AUC) values of 0.532 (95% CI: 0.392–0.672) for women with endometriosis and 0.456 (95% CI: 0.372–0.540) for those without endometriosis. In conclusion, our findings suggest that no specific endometrial thickness threshold above 6 mm is associated with higher LBRs in women with or without endometriosis undergoing artificial FET cycles. Further studies are needed to clarify the role of endometriosis—and the commonly associated adenomyosis—in the eutopic endometrium and its impact on endometrial receptivity.