Midluteal serum estradiol levels are associated with live birth rates in hormone replacement therapy frozen embryo transfer cycles: a cohort study

Objective: To study whether midluteal serum estradiol (E2) levels are associated with the live birth rate in hormone replacement therapy frozen embryo transfer (HRT-FET) cycles in patients with optimal midluteal serum progesterone (P4) levels. Design: Observational prospective cohort study. Setting: Public fertility clinic. Patients: A total of 412 women had an HRT-FET cycle single blastocyst transfer from January 2020 to November 2022. Intervention: The HRT-FET cycle priming regimen included oral E2 (6mg/24 h) administered in the evening, followed by vaginal P4 (400mg/12 h). Serum E2 and P4 levels were measured using a standardized method, 2-4 hours after the latest P4 administration and 9-14 hours after E4 administration on the day of blastocyst transfer, day 6 of P4 administration. Patients with serum P4 levels (< 11 ng/mL [35 nmol/L]) on the day of transfer received additional rectal P4 (400mg/12 h). No additional E2 dose was administered. Main Outcome Measures: The primary outcome was the live birth rate (LBR) in relation to E2 levels at blastocyst transfer day. Results: The optimal serum E2 levels correlating with ongoing pregnancy were >= 292 pg/mL and <409 pg/mL (>= 1,070 pmol/L and <1,500 pmol/L). The LBR was 59% (60/102) when E2 levels were within this range, whereas a significantly lower LBR of 39% (101/260) was seen in patients when E2 levels were <292 pg/mL (<1,070 pmol/L) and of 28% (14/50) when E2 levels were >= 409 pg/mL (>= 1,500 pg/mL). In a logistic regression analysis, adjusting for serum P4 level >= 11 ng/mL or <11 ng/mL (>= 35 nmol or <35 nmol/L) on the day of transfer, body mass index, age at oocyte retrieval, day 5 or 6 vitrified blastocysts, and blastocyst score, the adjusted risk difference of live birth was -0.21 (-0.32; -0.10) when the E2 level was <292 pg/mL (<1,070 pmol/L) and -0.31 (-0.45; -0.18) when the E2 level was >= 409 pg/mL (>= 1,500 pmol/L) compared with E2 levels >= 292 pg/mL and <409 pg/mL (>= 1,070 and <1,500 pmol/L). Importantly, only 25% of patents had optimal levels. Conclusion: The study shows a signi fi cant association between serum E2 levels and reproductive outcomes in an HRT-FET cohort in which optimal serum P4 levels were secured. Midluteal serum E2 levels are associated with the LBR in HRT-FET cycles, and E2 levels should neither be too high nor too low.