Zhou R, Zhang X, Huang L, et al. - Researchers conducted this retrospective cohort study with the aim to determine if levels of serum estradiol prior to progesterone administration in artificial endometrial preparation (AEP) of frozen blastocyst transfer affect live birth rate (LBR). Division of a total of 3,857 frozen-thawed blastocyst transfer cycles was done into three groups: < 200 pg/ml (n = 1676), 200–399 pg/ml (n = 1296), and ≥ 400 pg/ml (n = 885), based on the 25th (182.3 pg/ml) and 75th percentile (390.2 pg/ml) of serum E2 level prior to progesterone administration. Findings revealed correlation of high serum E2 levels prior to progesterone administration in AEP with reduced LBR after frozen-thawed blastocyst transfer.