High dose gonadotropin stimulation increases endometrial thickness but this gonadotropin induced thickening does not have an effect on implantation.

INTRODUCTION Endometrial thickness <8mm is related with lower pregnancy rates. This raises the question if endometrial thickness can be increased by gonadotropin stimulation to increase estradiol (E2) concentration and if such an artificial thickening of the endometrium has an effect on implantation. A model to address this question is the comparison of endometrial thickness and outcome parameters in conventional gonadotropin stimulated IVF (cIVF) compared to unstimulated natural cycle IVF (NC-IVF). MATERIAL AND METHODS Retrospective study including 235 cIVF and 616 NC-IVF cycles without embryo selection and with fresh transfer on day 2 and 3 from 2015 to 2019. Endometrial and E2 measurements were included and analysed between day -4 and -2 (0 = day of aspiration). The effects of E2 on endometrial thickness, endometrial growth and the effect of endometrial thickness on implantation rates and live births were analysed. RESULTS Endometrial thickness was found to be higher in cIVF compared to NC-IVF (p < 0.001). On day -2, the day when ovulation was triggered, mean endometrial thickness was 9.75 ± 2.05mm and 8.12 ± 1.66mm, respectively. The increase in endometrial thickness slowed down with increasing E2 concentrations (time x estradiol concentration: -0.19, p = 0.010). Implantation rates were not significantly different in cIVF and NC-IVF cycles (clinical pregnancy rate: 19.1% vs. 15.4% p = 0.2; live birth rate: 12.8% vs. 11.7%, p = 0.8). CONCLUSIONS Endometrial growth dynamic is different and endometrium is thicker in cIVF compared to NC-IVF. Pregnancy and live birth rates are not different. Gonadotropin induced thickening of the endometrium does not appear to improve implantation