Background. The association between serum progesterone (P) levels, measured on the day of ovulation trigger, and the outcome of in vitro fertilization cycles, has been one of the major controversies in the field of ovarian stimulation endocrinology. Objective. The aim is to review the dataset of a single tertiary care academic center in order to evaluate the impact of premature high serum P levels on the outcome of embryo transfer (ET) cycles in terms of Clinical Pregnancy Rates (CPR) and Live Birth Rates (LBR). Furthermore, the present study retrospectively investigates whether the transfer on day 5 post-fertilization may improve the CPR and the LBR in patients with P rise Methods. This is a single-center retrospective cohort study. All fresh ET both at cleavage stage and blastocyst stage, performed from January 2012 to December 2016, were included. Main clinical features were analyzed and the impact of P level on CPR and LBR was explored using a multivariable logistic regression. Results. During the study period 8,034 ET were performed: 7,597 cleavage-stage transfers and 437 blastocyst transfers. Serum P levels demonstrated to be inversely related to CPR (OR 0.72, p<0.001) and LBR (OR 0.73, p<0.001). The decrease already started when P levels were ³1 ng/mL, and a further decrease was observed with P levels ³1.75 ng/mL. Analyzing the day of embryo transfer: in the day 3 group, a P level ≥1ng/mL significantly reduced CPR and LBR. With a P level ≥1.75 ng/mL CPR and LBR decreased even more. The relationship between serum P level and CPR and LBR in the day 5 group instead is not particularly evident when the values are <1.75 ng/mL; on the other hand, CPR and LBR are half their value when P level is ≥1.75 ng/mL. To evaluate eventual differences in CPR depending on different P thresholds, the analysis was subsequently performed separately in different subgroups. In the cleavage ET subgroup patients with good prognosis, the effect of P increase was found to be significant for P values >1 ng/mL, while in patients with worse prognosis it appeared to be significant only for P values ≥1.75 ng/mL. In the blastocyst ET subgroup, the negative effect of P elevation on LBR and CPR was reported only if P was >1.75 ng/mL. CPR and LBR in thawed cycles resulted statistically significantly higher than in fresh cycles. Considering the cleavage subgroup, the thawed cycles group resulted in better CPR and LBR, even compared with patients with P level <1 ng/mL. In the blastocyst group, no significant difference resulted between thawed and fresh cycles, independently of P levels. Conclusion. High P levels decrease CPR as well as LBR in both cleavage and blastocyst ET. Interestingly, a different impact of P levels elevation was observed in different patient categories. In the cleavage group, for P levels below 1.75 ng/mL, it could be recommended to wait until day 5 for ET, and if P level is ≥1.75 ng/mL it should be considered to freeze all embryos and postpone the ET until P levels normalize.