The relationship between serum estradiol and progesterone levels one day before frozen embryo transfer and pregnancy rates in artificially prepared frozen embryo cycles: are there any threshold serum hormone levels to predict pregnancy in luteal support by the vaginal and subcutaneous route combined?

Objective: To investigate the potential influence of serum estradiol (E₂) and progesterone (P₄) levels, measured one day before artificially prepared frozen embryo transfer (FET), on pregnancy rates in women who received combined vaginal and injectable P₄.

Material and methods: This retrospective cohort study analyzed the association between serum E₂ and P₄ levels on the day before FET in 167 cases prepared with hormone replacement therapy between February 2022 and October 2023. The primary outcomes assessed were the pregnancy and live birth rates. We modeled a cut-off serum value based on luteal support for pregnancy. Luteal support was through a combination of vaginal suppositories and subcutaneous injections. Multivariate logistic regression was used to test relationships between pregnancy outcomes and independent variables. Cut-off values were evaluated using receiver operating characteristic (ROC) analysis and percentile analysis.

Results: No significant relationships were found between serum E₂ or P₄ levels on the day before FET and pregnancy rates. The mean E₂ level was 169.0±51.9 pg/mL for individuals who achieved conception and 177.7±56.9 pg/mL for individuals who did not conceive (p=0.45). The corresponding values for P₄ were 28.1±18.4 ng/mL and 31.2±25.4 ng/mL, respectively (p=0.73). No differences were observed in body mass index (BMI) or endometrial thickness between the groups. Cut-off values for predicting pregnancy using E₂ and P₄ could not be determined using ROCs. However, no one in the lowest 10^th percentile of serum P₄ levels conceived (range 10.0-15.6 ng/mL). When multivariate logistic regression was used, this finding lost significance suggesting that low serum levels are related to age, BMI, and/or other factors.

Conclusion: In artificially prepared FET cycles, the serum E₂ and P₄ levels one day before embryo transfer do not significantly affect pregnancy rates in women with serum E₂ levels between 150-300 pg/mL and P₄ between 10-40 ng/mL when ROC was used for evaluation. However, percentile analysis suggests that serum P₄ levels should be more than 15.6 ng/mL when combined injectable and vaginal P₄ is used for programmed FET. Although this finding may be due to the confounding effects of age, BMI, and other factors affecting steroid metabolism, when controlled for in the multivariate logistic regression.