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 (E) and progesterone (P) levels, measured one day before artificially prepared frozen embryo transfer (FET), on pregnancy rates in women who received combined vaginal and injectable P.

MATERIAL AND METHODS

This retrospective cohort study analyzed the association between serum E and P 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 E or P levels on the day before FET and pregnancy rates. The mean E 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 P 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 E and P could not be determined using ROCs. However, no one in the lowest 10 percentile of serum P 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 E and P levels one day before embryo transfer do not significantly affect pregnancy rates in women with serum E levels between 150-300 pg/mL and P between 10-40 ng/mL when ROC was used for evaluation. However, percentile analysis suggests that serum P levels should be more than 15.6 ng/mL when combined injectable and vaginal P 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.