Romanski PA, Bortoletto P, Liu YL, et al. - Researchers sought to determine the effect of the length of follicular phase estradiol exposure and the total length of the follicular phase on pregnancy and live birth outcomes in natural frozen embryo transfer (FET) cycles. In this retrospective cohort study, infertile patients who underwent their first natural frozen autologous Day-5 embryo transfer cycle were included. Based on the cohorts’ median number of days from the estradiol level of > 100 pg/ml before the LH surge, they stratified patients into two groups: Group 1 (≤ 4 days; n = 1,052 patients) and Group 2 (> 4 days; n = 839 patients). In addition, patients were divided into two groups based on the cohorts’ median cycle day of LH surge: Group 1 (follicular length ≤ 15 days; n = 1,287 patients) and Group 2 (follicular length > 15 days; n = 1,071 patients). Outcomes revealed worse pregnancy and live birth outcomes in correlation with an estradiol level > 100 pg/ml for ≤ 4 days including the LH surge day; however, there was no correlation of the total length of the follicular phase with pregnancy and live birth outcomes.