Serum progesterone is a test to measure the amount of progesterone in the blood. Progesterone is a hormone produced mainly in the ovaries. Progesterone plays a key role in pregnancy. It is produced after ovulation in the second half of the menstrual cycle. It helps make a woman's uterus ready for a fertilized egg to be implanted. It also prepares the uterus for pregnancy by inhibiting the uterine muscle to contract and the breasts for milk production.