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%;  = .046, and 24.0 vs. 18.6%;  = .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,  = .019) for the endometrium and odds ratio 1.95 (95%CI 1.31-2.9; = .001) to the protocol used. Endometrial thickness might predict crown-rump length (CRL) discordancy with odds ratio 4.61 ( = .001; 95% CI 1.42-14.92). Compared with group B, Group A had more cases of overt discordancy (13.3 vs. 4%;  = .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.

Summary: Endometrial thickness and preparation improved pregnancy rate in FET cycles and significantly greater crown-rump length discordancy was observed with thinner endometria.