Progesterone rise on the day of human chorionic gonadotropin administration impairs pregnancy outcome in day 3 single-embryo transfer, while has no effect on day 5 single blastocyst transfer.

Four hundred eighty-two patients undergoing single ET with GnRH-antagonist/recFSH protocol were analyzed. The incidence of premature luteinization (P above 1.5 ng/mL on the day of hCG administration) was 18.2%. Even modest rises of P in the follicular phase have detrimental effect on the implantation potential of a good-quality cleavage stage embryo. On the contrary, premature luteinization in the blastocyst subgroup had no effect on the pregnancy outcome.