Influence of body mass index on the relationship between endometrial thickness and pregnancy outcome in single blastocyst frozen embryo transfer cycles.

To date there is limited published data assessing whether body mass index (BMI) influences endometrial thickness (ET) and whether this impacts on pregnancy outcomes in single blastocyst FET cycles. The objective of this study, therefore, was to examine the relationship between BMI and ET on the outcome of single blastocyst FET cycles over a five-year period from 2012 until 2016. Patient age, BMI, endometrial pattern and ET prior to FET were recorded. Pregnancy outcomes included: implantation rate, clinical pregnancy rate and live birth rate. A total of 464 cycles met the inclusion criteria and the female age was 36.0 ± 3.0 years (mean ± SD). The mean ± SD BMI was 23.3 ± 3.1 kg/m and median ± SD ET was 8.1 ± 1.5 mm. BMI and ET were modestly correlated (Pearson  = 0.244) and there was an association between higher BMI category and higher median ET (7.2, 8.0, 8.3, 8.9 mm;  < 0.001). However, there was no association between ET and pregnancy outcome, either unadjusted, or adjusted for BMI, age, endometrial pattern or embryo quality. The data suggests that although ET increases with increasing BMI, there are no differences in cycle outcome. Importantly, this implies that an ET <8 mm may not jeopardize pregnancy outcome in women with lower BMI. The development of a norm referenced test for BMI and ET may prove to be a helpful adjunct in the clinical IVF setting.