A progesterone test measures the amount of the hormone progesterone in a blood sample. Progesterone is a female hormone produced by the ovaries during release of a mature egg from an ovary (ovulation). Progesterone helps prepare the lining of the uterus (endometrium) to receive the egg if it becomes fertilized by a sperm. If the egg is not fertilized, progesterone levels drop and menstrual bleeding begins. During pregnancy, the placenta also produces high levels of progesterone, starting near the end of the first trimester and continuing until the baby is born. Levels of progesterone in a pregnant woman are about 10 times higher than they are in a woman who is not pregnant.