The effect of endometrial thickness and pattern on the success of frozen embryo transfer cycles and gestational age accuracy

Objective To evaluate the relationship between endometrial thickness measured before embryo transfer, and pregnancy outcomes in frozen-thawed embryo transfer (FET). Methods We retrospectively analyzed outcomes of all consecutive FET cycles, from January 2012 to August 2018. Based on ROC analysis for endometrial thickness, we found 8 mm was a reliable cutoff point to predict pregnancy prior to embryo transfer. Accordingly, the cycles were divided into Group A: cycles with endometrial thickness <= 8 mm and Group B: > 8 mm. Results Group A included 485 FET cycles and group B included 626 cycles. Compared with group A, Group B had significantly higher chemical and clinical pregnancy rates (30.3 vs. 24.6%;p = .046, and 24.0 vs. 18.6%;p = .036), respectively. In multivariate analysis, endometrial thickness and the protocols used were the only parameters influencing the chance to achieve pregnancy, with odds ratio 1.54 (95%CI 1.07-2.22,p = .019) for the endometrium and odds ratio 1.95 (95%CI 1.31-2.9;p= .001) to the protocol used. Endometrial thickness might predict crown-rump length (CRL) discordancy with odds ratio 4.61 (p = .001; 95% CI 1.42-14.92). Compared with group B, Group A had more cases of overt discordancy (13.3 vs. 4%;p = .016). Conclusions For patients undergoing FET cycles, endometrial thickness and treatment protocol may predict the chemical and clinical pregnancy rates, as well as CRL discordancy. Endometrial thickness and preparation improved pregnancy rate in FET cycles and significantly greater crown-rump length discordancy was observed with thinner endometria.