Serum progesterone is a test to measure the amount of progesterone in the blood. Progesterone is a hormone produced mainly in the ovaries. In women, progesterone plays a vital role in pregnancy. After an egg is released by the ovaries (ovulation), progesterone helps make the uterus ready for implantation of a fertilized egg. It prepares the womb (uterus) for pregnancy and the breasts for milk production. Men produce some amount of progesterone, but it probably has no normal function except to help produce other steroid hormones.