A progesterone test measures the level of progesterone in the blood. Progesterone is a

hormone made by a woman's ovaries. Progesterone plays an important role in pregnancy. It

helps make your uterus ready to support a fertilized egg. Progesterone also helps prepare

your breasts for making milk.Progesterone levels vary during a woman's menstrual cycle.

The levels start out low, then increase after the ovaries release an egg. If you become

pregnant, progesterone levels will continue to rise as your body gets ready to support a

developing baby. If you do not become pregnant (your egg is not fertilized), your

progesterone levels will go down and your period will begin.