Purpose: The purpose of this study is to investigate whether progesterone (P4) levels on the day of frozen-thawed embryo transfer (FET) to a hormonally prepared endometrium correlate with pregnancy outcomes.
Methods: This is a large retrospective cohort analysis comprising of N = 2010 FETs. In these cycles, P4 levels on the day of transfer were assessed in relation to pregnancy outcomes. A threshold of 10 ng/mL was used to simulate currently accepted levels for physiological corpus luteal function. Biochemical pregnancy, clinical pregnancy, and live birth rates were compared between those with P4 levels above and below this threshold. Analyses using transfer day P4 thresholds of 5 ng/mL and 20 ng/mL were then completed to see if these could create further prognostic power.
Results: When comparing FET outcomes in relation to P4 levels < 10 ng/mL and ≥ 10 ng/mL, we observed no differences in biochemical pregnancy rates (39.53% vs. 40.98%, p = 0.52), clinical pregnancy rates (20.82 vs. 22.78, p = 0.30), and live birth rates (14.25 vs. 16.21 p = 0.23). In patients whose P4 met the threshold of 20 ng/mL, there was similarly no statistically significant improvement in pregnancy outcomes. While there was no difference for biochemical or clinical pregnancy rates, a statistically significant improvement in live birth rates was observed for those with a transfer day P4 level ≥ 5 ng/mL.
Conclusions: We demonstrated that P4 levels at or above 10 ng/mL on the day of FET do not confer a statistically significant improvement in pregnancy outcomes. P4 below 5 ng/mg was associated with lower live birth rates suggesting that there is a threshold below which it is difficult to salvage FET cycles.
Keywords: Assisted reproductive technology; Embryo transfer; Frozen; Progesterone.