Thomsen LH, et al. - Researchers performed a prospective multicenter cohort study of 602 women undergoing IVF treatment investigating the impact of early and mid-luteal serum progesterone (P4) levels on the attempt of a live birth following IVF treatment and fresh embryo transfer. Among these women, the optimal chance of pregnancy was achieved with serum P₄ levels of 60–100 nmol/l in the early luteal phase whereas the optimal P₄ level during the mid-luteal phase was 150–250 nmol/l. Results revealed a reduction in the chance of a live birth following IVF treatment with fresh embryo transfer in association with P4 levels lower and especially higher than these ranges in the early and mid-luteal phase.