Do mid-luteal estradiol levels impact ongoing pregnancy rate in hormone replacement transfer frozen embryo transfer (hrt-fet) cycles?

The aim of the study was to evaluate the impact of mid-luteal serum estradiol (E2) levels on the ongoing pregnancy rate (OPR) week 12 in Hormone Replacement Therapy Frozen Embryo Transfer (HRT-FET) cycles with a personalized luteal phase progesterone support (LPS), securing optimal serum progesterone (P4) levels. Mid-luteal E2 levels, unlike P4 levels, until now received little attention. An observational interventional prospective cohort study in a public fertility clinic, including 412 HRT-FET cycles conducted from January 2020 to November 2022. All patients underwent HRT from 2nd cycle day, including oral estradiol (6mg/24hours) administered in the evening, followed by vaginal micronized progesterone 400mg BID (7 am, 7 pm). On the 6th progesterone treatment day single embryo transfer was performed, and E2 levels and P4 levels were measured in a standardized manner (9am – 11am). Patients with serum P4 <11ng/ml received additional rectal progesterone (400mg/12hours) for rescue to secure optimal P4 levels from the ET day. Patients with P4 ≥11ng/ml on the ET day continued the standard LPS. In case of pregnancy all patients continued LPS until 10th week of gestation. No PGT-A was performed. The cut-off levels were calculated using the Youden Index which is the cut-off with the highest sensitivity and specificity. The overall positive hCG rate, OPR week 12, and total pregnancy loss rate were 65% (266/412), 43% (179/412) and 33% (87/266), respectively. The mean E2 level was 265 ±115pg/ml and no correlation between E2 levels and P4 levels (r= -0.013, p=0.79) on the ET day was found. Moreover, no correlation was seen between E2 levels and age (r=0.06, p=0.25) or E2 levels and BMI (r=-0.06 p=0.27). E2 level quartiles indicated a non-linear relation between E2 levels and OPR and the optimal cut-off level for an ongoing pregnancy was seen in patients with E2 levels ≥292pg/ml and ≤409pg/ml. Thus, the OPR for patients with E2 levels ≥292pg/ml ≤409pg/ml was 61% (62/102) which was significantly higher compared to patients with E2 levels <292pg/ml, 40% (103/260, p<0.001) and also significantly higher compared to patients with E2 levels >409pg/ml, 28% (14/50, p<0.001). Adjusting for the two progesterone level groups (≥11ng/ml or <11ng/ml) on the ET day, BMI, Age at oocyte retrieval, transfer of day 5 or 6 blastocysts and blastocyst score, the risk difference of OPR was -0.22 [-0.33; -0.11] with E2 levels <292pg/ml and -0.33 [-0.46; -0.20] with E2 levels >409pg/ml compared to an E2 level ≥292pg/ml and ≤409pg/ml. Mid-luteal E2 levels in an HRT-FET cycle impact the OPR in a bell-shaped manner. Thus, E2 levels should neither be too high nor too low to obtain the highest OPR.