Baseline Endometrial Thickness or Endometrial Thickness Change in Response to Estrogen Is Not Predictive of Frozen Embryo Transfer Success in Medicated Cycles

There is some consensus that endometrial thickness (EMT) needs to be at least 7 mm on day of embryo transfer. However, the predictive role of baseline EMT and EMT change in response to estrogen is largely unknown. The objective of this study was to evaluate the role of endometrial thickness in frozen embryo transfer (FET) cycles. We analyzed the association of baseline endometrial thickness (EMTb-Day 3 of cycle) and endometrial thickness change (EMT Delta-from baseline to start of progesterone supplementation) with FET success in 121 cycles. We also investigated whether baseline estradiol levels and body mass index (BMI) are associated with EMTb. No difference was observed in EMTb and EMT Delta in cycles resulting in clinical pregnancy compared to unsuccessful transfers (5.1 +/- 2.2 mm vs 5.0 +/- 1.9 mm;p = 0.92, and 4.7 +/- 2.4 mm vs. 4.4 +/- 2.4 mm;p = 0.56). When 7 mm cut-off was used, endometrial thickness on the day of start of progesterone supplementation (EMTp) was also not different between groups (9.8 +/- 2.9 mm vs. 9.4 +/- 2.5 mm;p = 0.50). Multivariable logistic regression models did not demonstrate any predictive value of EMTb, EMTp, or EMT Delta in predicting success of FET cycles (p = 0.92,p = 0.80, andp = 0.84, respectively). There was no significant correlation between EMTb and baseline estradiol levels (r = -0.001;p = 0.985). BMI showed statistically significant weak positive linear relationship with EMTb (r = +0.29;p = 0.002). Our study did not demonstrate any significant relationship between baseline endometrial thickness or endometrial thickness change and clinical pregnancy rates in frozen embryo transfer cycles. Significant positive linear relationship of BMI with baseline endometrial thickness, despite no correlation between baseline estradiol and EMTb, points to the role of possible other mechanism affecting EMT besides estradiol in obese patients.