Analysis of endometrial thickness threshold and optimal thickness interval measured by transvaginal ultrasound in blastocyst hormone replacement freeze-thawed embryo transfer

Objective To investigate the effect of endometrial thickness(EMT) on the clinical outcome of blastocyst hormone replacement freeze-thawed embryo transfer (HRT-FET) on the first progesterone day, and to analyze the threshold and optimal thickness interval corresponding to ideal clinical pregnancy rate by statistical method. Methods The endometrial preparation protocols of 2 825 blastocyst HRT-FET cycles from January 2013 to December 2016 in Henan Provincial People′s Hospital and the Second Hospital of Hebei Medical University were studied retrospectively. According to EMT on the first progesterone day, they were divided into 5 subgroups: group Q1(EMT: 3.5-7.9 mm), group Q2(EMT: 8.0-8.9 mm), group Q3(EMT: 9.0-9.5 mm), group Q4(EMT: 9.6-10.7 mm), group Q5(EMT: 10.8-21.0 mm). Univariate analysis, classification multivariate Logistic regression analysis, curve fitting and threshold effect analysis were used to investigate the effect of endometrial thickness on clinical outcome of blastocyst HRT-FET. Results Group Q1 was set as the control group in classification multivariate Logistic regression analysis, after adjusting for confounding factors, the clinical pregnancy rate and live birth rate in other groups were higher than the control group. The clinical pregnancy rate and live birth rate in group Q3 and Q4 were significantly increased and the differences were statistically significant(all P<0.05). The cut-off value of the endometrial thickness was 9.6 mm. When endometrial thickness was less than 9.6 mm, with 1 mm increase of endometrial thickness, the clinical pregnancy rate increased by 23%(OR=1.23, 95%CI=1.11-1.36) and the live birth rate increased by 21%(OR=1.21, 95%CI=1.10-1.33). When the endometrial thickness was thicker than the threshold, the clinical pregnancy rate did not increase significantly(OR=0.92, 95%CI=0.84-1.02), and the live birth rate showed a downward trend(OR=0.88, 95%CI=0.81-0.96). Conclusions In the blastocyst HRT-FET cycle, endometrial thickness showes a curvilinear relationship with clinical outcome. The optimal endometrial thickness range for ideal clinical outcome is 9.0-11.0 mm. Key words: Ultrasonography, transvaginal; Freeze-thawed embryo transfer; Endometrial thickness; Clinical pregnancy rate; Live birth rate