OBJECTIVE: To analyze the differences in pregnancy rates (PRs) from the transfer of fresh and cryopreserved embryos from the same cohort of oocytes based on serum P levels on the day of hCG administration and the day after. DESIGN: Retrospective analysis. SETTING: Infertility patients stimulated for IVF-ET in an academic center. PATIENT(S): Three hundred thirty-three patients with fresh transfer and at least one transfer of cryopreserved embryos from the same cohort of recruited oocytes. All stimulations were down-regulated with a GnRH agonist in a long protocol before gonadotropin stimulation. MAIN OUTCOME MEASURE(S): Clinical PR. RESULT(S): The clinical PR in fresh cycles was 24% for the P < or = 0.9 ng/mL group (group A; conversion factor to SI unit, 3.18) and 34% for the P > 0.9 ng/mL group (group B). Group B patients were younger, received fewer ampules of gonadotropins, had higher peak E2 levels, and had more mature oocytes. There were no significant differences in the P levels on the day of hCG between patients who conceived in both fresh and cryopreserved cycles and any other combination of pregnancy outcome sequence. CONCLUSION(S): These findings suggest that serum P level cutoffs, on the day of hCG and the day after, as a means of making clinical decisions with respect to cancelling the fresh transfer and cryopreservation of all embryos for future transfer should be questioned.