The effect of endometrial thickness on pregnancy outcomes of frozen-thawed embryo transfer cycles which underwent hormone replacement therapy.

Objective: To investigate the impact of endometrial thickness on the embryo transfer(ET) day on the clinical pregnancy outcomes of frozen-thawed embryo transfer cycles which have undergone hormone replacement therapy(HRT-FET).

Methods: A total of 10,165 HRT-FET cycles performed between January 2013 to December 2017 in the Reproductive Medicine Center of Henan Provincial People's Hospital were studied retrospectively. All patients were grouped according to their endometrial thickness on the ET day (each group having an increment of 1mm between two neighboring groups). Multivariate regression analysis, curve fitting and threshold effect analysis were performed on all data.

Results: After adjusting for the age, duration of infertility, body mass index(BMI), infertility type and number and type of embryos transferred, a significant correlation was observed to be between the endometrial thickness and implantation rates (aOR: 1.08; 95% CI: 1.06-1.10, p < 0.0001), clinical pregnancy rate(aOR: 1.10; 95% CI: 1.07-1.14, p < 0.0001)and live birth rate (aOR: 1.09; 95% CI: 1.06-1.12, p < 0.0001). The numerical value of the cut-off point for the endometrial thickness was 8.7 mm. When the endometrial thickness was less than 8.7 mm, with each additional 1 mm of endometrial thickness, the implantation rate increased by 32%, the clinical pregnancy rate increased by 36%, and the live birth rate increased by 45%.

Conclusions: In the HRT-FET cycles, the optimal live birth rate would be obtained when the endometrial thickness remains within the range of 8.7-14.5 mm. If the endometrium is too thin or too thick, the live birth rate will be reduced.