Increasing Endometrial Thickness Beyond 8 mm Does Not Alter Clinical Pregnancy Rate After Single Euploid Embryo Transfer

The aim of this study was to investigate if variation in endometrial thickness affects clinical pregnancy and live birth rates among patients undergoing single euploid embryo transfer (SET). A retrospective review of IVF cycles performed at a single private fertility institution between 2015 and 2020 was performed. Patients with normal uterine anatomy undergoing their first SET of a euploid embryo undergoing their first cycle at the center were included, for a total of 796 cycles. Endometrial thickness was measured by transvaginal ultrasound following 10–14 days of estradiol exposure. Specific infertility diagnoses did not significantly impact endometrial lining thickness with means across diagnoses ranging from 9.3 to 11.0 mm. Endometrial thickness was grouped into five categories: < 8 mm, 8–10 mm, 10–13 mm, 13–15 mm, and ≥ 15 mm. Using 8–10 mm as the reference group, the odds ratio of live birth was 0.5, 1.22, 1.05, and 1.05 for < 8 mm, 10–13 mm, 13–15 mm, and ≥ 15 mm groups, respectively. Risk of first trimester miscarriage was equivalent across groups. There was a trend toward an increased rate of biochemical pregnancies in patients with a < 8 mm and ≥ 15 mm endometrium; however, this was not statistically significant. The clinical pregnancy and live birth rate were lowest in patients with < 8-mm endometrial thickness. For single euploid embryo transfers, an endometrial lining greater than or equal to 8 mm confers optimal live birth rates following a medicated FET cycle. These data confirm the findings of prior studies in fresh embryo transfers without the confounders of supraphysiologic ovarian hormone concentrations and genetically untested embryos.