This test measures the level of progesterone in the blood. Progesterone is a steroid hormone whose main role is to help prepare a woman’s body for pregnancy; it works in conjunction with several other female hormones.

On a monthly basis, the hormone estrogen causes the endometrium (the lining of the uterus) to grow and replenish itself, while a surge in lutenizing hormone (LH) leads to the release of an egg from one of two ovaries. A corpus luteum then forms in the ovary at the site where the egg was released and begins to produce progesterone. This progesterone (supplemented by small amounts produced by the adrenal glands) stops endometrial growth and readies the uterus for the possible implantation of a fertilized egg.

If fertilization does not occur, the corpus luteum degenerates, progesterone levels drop, and menstrual bleeding begins. If a fertilized egg is implanted in the uterus, the corpus luteum continues to produce progesterone. After several weeks, the placenta replaces the corpus luteum as the main source of progesterone, creating relatively large amounts of the hormone throughout the rest of a normal pregnancy.