Optimal endometrial thickness to maximize live births and minimize pregnancy losses: Analysis of 25,767 fresh embryo transfers

Research question: What is the association of endometrial thickness with pregnancy losses and live births in IVF treatment and the optimal threshold that optimizes the IVF outcome? Design: Data were analysed from 25,767 IVF cycles from centres of the CARE Fertility Group in the UK between 2007 and 2016. Transvaginal ultrasound was conducted to measure the maximum endometrial thickness during gonadotrophin stimulation. Live birth rates were per embryo transfer. Pregnancy loss rates included the combination of biochemical and clinical pregnancy losses. Results: The live birth rate was 15.6% with 5 mm or less endometrial thickness and gradually increased to 33.1% with an endometrial thickness of 10 mm. On the other hand, the pregnancy loss rate was 41.7% with 5 mm or less endometrial thickness and gradually decreased to 26.5% with an endometrial thickness of 10 mm. Statistical modelling for optimal endometrial thickness threshold found 10 mm or more maximized live births and minimized pregnancy losses. This association was independent after adjusting for confounders such as age, oocyte number, number of transferred embryos, ovarian stimulation protocol and embryo quality for live births (crude RR 1.27; 95% CI 1.21 to 1.33; Adjusted RR 1.18; 95% CI 1.12 to 1.23) and pregnancy losses (crude RR 0.83; 95% CI 0.77 to 0.89; adjusted RR 0.86; 95% CI 0.8 to 0.92). Conclusions: Endometrial thickness is strongly associated with pregnancy losses and live births in IVF, and the optimal endometrial thickness threshold of 10 mm or more maximized live births and minimized pregnancy losses.