When should progesterone be tested?

Progesterone levels rise after ovulation and peak five to nine days after your luteal phase–which occurs during the second half of the menstrual cycle, after ovulation occurs–so progesterone level is usually checked six to eight days after you ovulate (about day 21 of a day 28 cycle).

Why is progesterone test done on day 21?

Checking on “Day 21” can confirm whether the progesterone and estradiol levels have improved and are adequate to support a pregnancy. During treatment cycles, your fertility specialist may also check your peak luteal progesterone and estrogen levels.