Mid-luteal serum estradiol levels are associated with Live birth rates in Hormone Replacement Therapy Frozen Embryo Transfer (FET) cycles – a cohort study

Objective To study whether mid-luteal serum estradiol (E2) levels are associated with the Live Birth Rate in Hormone Replacement Therapy FET cycles in patients with optimal mid-luteal serum progesterone (P4) levels. Design Observational prospective cohort study of 412 women having a Hormone Replacement Therapy FET single blastocyst transfer from January 2020 to November 2022. Subjects The Hormone Replacement Therapy FET priming regimen included oral estradiol (6mg/24h) administered in the evening, followed by vaginal progesterone (400mg/12h). Serum E2 and P4 levels were measured in a standardized manner, 2-4 hours after the latest progesterone administration and 9-14 hours after estradiol administration on the day of blastocyst transfer, day 6 of progesterone administration. Patients with serum P4 <11ng/mL (35nmol/l) on the day of transfer received additional rectal progesterone (400mg/12h). No additional estradiol was administered. Main Outcome Measures The primary outcome was Live Birth Rate in relation to E2 levels at blastocyst transfer day. Results The optimal serum E2 range correlating with ongoing pregnancy was ≥292pg/ml <409pg/ml (≥1070pmol/l and <1500pmol/l). The Live Birth Rate was 59% (60/102) if E2 levels were within this range, whereas a significantly lower Live Birth Rate of 39% (101/260, p=0.001) was seen in patients if E2 levels were <292pg/ml (<1070pmol/l), and of 28% (14/50, p<0.001) if E2 levels were ≥409pg/ml (≥1500pg/mL). In a logistic regression analysis, adjusting for serum progesterone level ≥11ng/mL or <11ng/mL (≥35nmol or <35nmol/l) on the day of transfer, BMI, age at oocyte retrieval, day 5 or 6 vitrified blastocyst and blastocyst score, the adjusted risk difference (RD) of a live birth was -0.21 [-0.32; -0.10] when the E2 level was <292pg/mL (<1070pmol/l) and -0.31 [-0.45; -0.18] if the E2 level was ≥409pg/ml (≥1500pmol/l) compared to E2 levels ≥292pg/ml <409pg/ml (≥1070 <1500pmol/l). Importantly, only 25% of patents had optimal levels. Conclusion The study shows a significant association between serum E2 levels and reproductive outcomes in a Hormone Replacement Therapy FET cohort in which optimal serum progesterone levels were secured. Mid-luteal serum E2 levels are associated with Live Birth Rate in Hormone Replacement Therapy FET cycles and E2 levels should neither be too high nor too low.