Effect of the Endometrial Thickness on the Live Birth Rate: Insights From 959 Single Euploid Frozen Embryo Transfers Without a Cutoff For Thickness

ABSTRACT Although numerous studies have examined whether endometrial thickness (ET) independently affects the live birth rate (LBR) after an embryo transfer, contradictory conclusions have resulted in an unclear answer. It has been hypothesized that a thin endometrium decreases implantation rates possibly due to elevated oxygen concentration from spiral arteries; however, this mechanism has not been unequivocally demonstrated. This retrospective analysis aimed to investigate whether ET independently affects the LBR after a frozen embryo transfer cycle. All patients who underwent a single euploid blastocyst transfer between March 2017 and March 2020 at a single-assisted reproductive technology clinic were included. Live birth was defined as the delivery of a live fetus after 22 weeks of gestation. Programmed cycle (PC) or natural cycle (NC) for endometrial preparation was conducted at physician’s and patient’s discretion. The presence of a linear relationship between ET and LBR was assessed by conditional density plots (CDPs), and receiver operating characteristics (ROC) curve analyses were used to identify whether a threshold of the ET existed to predict the occurrence of live birth. The CDPs were analyzed for an optimal range of the ET that could be associated with a higher LBR, and the distribution of cycle characteristics and embryo quality in that range were compared with cycles outside of that range to identify confounders. Logistic regression models were constructed separately for the PC and NC. A total of 959 single euploid FETs were included in this analysis, of which 33% (n = 315) were NC and 67% (n = 644) were PC with an overall LBR of 47.1% (452/959). No linear relationship between the ET and LBR or threshold below which the LBR decreased perceivably was identified. In addition, ROC analysis did not suggest a predictive value of the ET for occurrence of live birth based on endometrial preparation or in the overall cohort. Similarly, logistic regression analyses showed no independent effect of the ET on LBR. The results of this study suggest that there is no linear relationship between the ET and LBR or a clinically significant cutoff for ET below which the LBR decreases in FET cycles. Significantly, the common practice of canceling embryo transfers when the ET is <7 mm is not supported by these data.