Higher serum progesterone level has no negative impact on live birth rate in frozen embryo transfer

Optimal synchronisation between the endometrium and the embryo is key to the success of frozen embryo transfers (FET). It is achieved by administering different doses of exogenous oestrogen and progesterone (P4). The negative impact of low levels of P4 on FET has been reported, but there is a lack of knowledge about which levels are most appropriate. We aimed to evaluate whether high serum P4 levels measured on FET day affect the probability of having a baby at home. This retrospective cohort study includes 7546 FET cycles performed with hormone replacement therapy (HRT) for artificial endometrial preparation. Patients were divided into three groups according to P level on FET day (ng/mL): P4 < 20(n = 6623), P4 ≥ 20–40 (n = 770) and P > 40 (n = 146). Female age was per group P4 < 20 = 38.7 ± 3.1, P ≥ 20–40 = 39.0 ± 3.0, and P4 > 40 = 38.1 ± 2.9 years old (p = 0.53). The group with the highest progesterone levels found lower ongoing pregnancy rates without statistical significance (56.3 % (P4 < 20) vs 56.8 % (P4 ≥ 20-<40) vs 51.4 % (P4 > 40); p = 0.5). Miscarriage rate was also higher, although not significantly: 16.2 % (P4 < 20) vs 15.0 % (P4 ≥ 20-<40) vs 18.0 % (P4 > 40) (p = 0.6). These findings were explored with an adjusted analysis. A higher, but not significant, odds of miscarriage was observed when P4 > 40 ng/ml, aOR = 1.14 (0.75–1.74) (p = 0.55) whereas no such association was found with P4 ≥ 20–40 ng/ml (aOR = 0.92 (0.74–1.15) (p = 0.46)). The probability of livebirth is similar when the patient was P4 > 40 (aOR = 0.77 (0.51–1.15) (p = 0.20)) than when P4 ≥ 20-<40 (aOR = 0.94 (0.79–1.11) (p = 0.47). In conclusion, women with elevated serum P4 levels on the day of FET after HRT do not find their probability of live birth impaired.