Figure 1.

Luteal phase scenarios in ART. No corpus luteum (upper panel): artificial cycle (HRT). Progesterone only emanates from the luteal support (exogenous progesterone). Serum progesterone level quickly reaches a plateau (estimated serum progesterone level with vaginal progesterone: 10–15 mg/ml), except for injected progesterone for which peaks are observed initially (dotted line). Serum progesterone level is not modified in the event of a pregnancy as hCG does not interfere with the exogenous progesterone. One or few corpora lutea (middle panel): after ovulation trigger, endogenous progesterone secretion is pulsatile and varies during the day (estimated serum progesterone level: 25–35 mg/ml). There might be a small endogenous progesterone gap (mainly in cycles with few CL, in between a possible luteal insufficiency and the taking over of the hCG from the pregnancy). Luteal support is indicated to cover that gap. Several corpora lutea (lower panel): after ovulation trigger, endogenous progesterone secretion is pulsatile and varies during the day (estimated serum progesterone level: about 40–80 mg/ml, but might vary according to the number of CL). There will be a large endogenous progesterone gap (in between the iatrogenic luteal insufficiency and the taking over. Luteal support is indicated to cover that gap.