Labarta E, Mariani G, Paolelli S, et al. - In view of their previous work establishing a correlation of serum progesterone (P) levels of < 9.2 ng/ml on the day of embryo transfer (ET) with significantly reduced ongoing pregnancy rate (OPR) in a sample of 211 oocyte donation recipients, researchers here examined if these results are applicable to all infertile patients under an artificial endometrial preparation cycle, regardless of the oocyte origin. Participants were patients ≤ 50 years of age with a triple-layer endometrium ≥ 6.5 mm and underwent transfer of one or two blastocysts. They analyzed a total of 1,150 patients who underwent treatment with own oocytes without preimplantation genetic testing for aneuploidies (PGT-A) (n = 184), own oocytes with PGT-A (n = 308) or donated oocytes (n = 658). A significantly lower OPR (36.6% vs 54.4%) and live birth rate (35.5% vs 52.0%) was reported among women with serum P levels < 8.8 ng/ml (30th percentile) relative to the rest of the patients. Overall findings revealed lower ongoing pregnancy rate (OPR) in both own or donated oocyte cycles in correlation with serum P levels < 8.8 ng/ml on the day of ET.