In women who are not pregnant, Progesterone prepares the endometrium, or lining of the uterus, for implantation by the fertilized egg. Progesterone Levels are lowest during menstruation, and remain low up until ovulation. This is the point approximately midway through a normal menstrual cycle when an egg is released from one follicle in the ovary. After releasing the egg, the follicle is known as the corpus luteum, and begins producing increasing levels of Progesterone. This Progesterone acts on the endometrium to make it thicker and richer in blood supply and nutrients to prepare for implantation by the fertilized egg. If the egg is not fertilized, Progesterone Levels drop rapidly, causing most of the endometrium to break down and leaving only a thin layer of tissue. The broken-down endometrium is shed during a womans period.

If fertilization takes place, the corpus luteum in the ovary continues to produce Progesterone during the first eight weeks of pregnancy. Once the embryo is implanted in the wall of the uterus, the placenta begins to form. From the eighth week of pregnancy onwards, the placenta becomes the main source of Progesterone. Normal Progesterone Levels are essential during pregnancy, particularly in the early stages when Progesterone helps stabilize the uterine muscles. As pregnancy progresses, Progesterone acts as a precursor for other hormones essential for normal fetal development. Progesterone also plays a role in preparing the breasts for milk production, but prevents the release of milk before delivery.

If Progesterone Levels are low, a woman may have difficulty conceiving or in maintaining a pregnancy. Low Progesterone Levels in early pregnancy are a major cause of miscarriage. In either of these conditions, Progesterone supplements may be prescribed to help a woman become pregnant or to maintain a pregnancy.