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.