Progesterone levels on pregnancy test day after hormone replacement therapy-cryopreserved embryo transfer cycles and related reproductive outcomes

Research question: Do serum progesterone levels determine ongoing pregnancy rates (OPR) in hormone replacement therapy frozen-thawed embryo transfer (HRT-FET) cycles? Design: A cohort study of 244 HRT-FET cycles from a Danish public fertility centre. Data from patients undergoing HRT-FET from January 2016 to December 2017 were extracted from a clinical database. All patients had transfer in HRT cycles of autologous embryos frozen on day 5 or 6. Endometrial preparation was performed using 6 mg oestradiol valerate daily from the second day of the cycle followed by vaginal micronized progesterone (90 mg/8 h). All patients had serum progesterone measurement during the artificial luteal phase. Results: The optimal cut-off for ongoing pregnancy was 35 nmol/l based on sensitivity analysis of different progesterone levels as a factor variable and its association with ongoing pregnancy. No significant differences regarding number of embryos transferred, embryo quality, age, body mass index (BMI) or smoking were found in the two groups of progesterone < 35 nmol/l and >= 35 nmol/l, respectively. A total of 51%of patients had a serum progesterone < 35 nmol/l. The range of all measurements was 0.3 to 110 nmol/l. The unadjusted OPR was significantly lower in the < 35 nmol/l group compared with the >= 35 nmol/l group (38% versus 51%; P = 0.04). A logistic regression analysis, adjusting for smoking, age, BMI, number of embryos transferred and blastocyst age showed a significant decrease in OPR when progesterone was < 35 nmol/l of 44% (95% confidence interval [CI] 35-54%) compared with = 35 nmol/l of 58% (95% CI 48-68%), risk difference of 14% (95% CI 2-26%, P = 0.02). Conclusions: Serum progesterone levels < 35 nmol/l decrease the chance of OPR in HRT-FET cycles.