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.

This is a single-center retrospective cohort study. Study duration period is from January 2012 to December 2016.

All fresh embryo transfer (ET) both at cleavage stage and blastocyst stage, performed in Humanitas Fertility Center during the study period, were included.

Inclusion Criteria:

• All fresh embryo transfers both at cleavage stage (Day 2 and Day3) and blastocyst stage (Day 5)
• Thawed embryo transfer deriving from supernumerary embryos cycles, by P increase, by OHSS, by other reasons (i.e inadequate endometrium, medical complications, etc)

Exclusion Criteria:

• thawed embryo transfers deriving from PGTA.

All fresh embryo transfers both at cleavage stage (Day 2 and Day3) and blastocyst stage (Day 5), and all thawed embryo transfer, except from the ones deriving from PGTA, performed from January 2012 to December 2016 in Humanitas Fertility Center were included.