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Summary The live birth rate following embryo transfer is comparable between spontaneous, stimulated and artificial cycles. However, the pregnancy loss rate appears elevated with hormonal therapy, possibly due to luteal insufficiency. This study aimed to determine whether the serum progesterone level on transfer day differed according to the endometrial preparation method for frozen embryo transfer (FET). Twenty spontaneous cycles (SC), 27 ovarian stimulation cycles (OS) and 65 artificial cycles (AC) were retrospectively studied from May to December 2019 in a single French hospital. The primary endpoint was the level of serum progesterone on the day of FET between the three endometrial preparation methods. The mean serum progesterone level on transfer day was 29.47 ng/ml in the OS group versus 20.03 ng/ml in the SC group and 14.32 ng/ml AC group (P < 0.0001). Progesterone levels remained significantly different after logistic regression on age and anti-Müllerian hormone (AMH) level. There was no significant difference in demographic and hormone characteristics (age, body mass index, embryo stage of embryo, type of infertility, basal follicle stimulating hormone, luteinizing hormone, estradiol and AMH levels), endometrial thickness, number and type of embryos transferred, duration of infertility, pregnancy rate, live birth rate and pregnancy loss rate. No difference was found in serum progesterone levels between clinical pregnancy with fetal heartbeat and no clinical pregnancy (no pregnancy or pregnancy loss, 17.49 ng/ml vs 20.83 ng/ml, respectively, P = 0.07). The lower serum progesterone level found on FET day in the AC group should be further investigated to see whether this difference has a clinical effect on the live birth rate.