Progesterone is a hormone produced primarily by females. Hormones are a group of chemicals secreted by special glands throughout the body. Hormones are carried by the bloodstream to target tissues, where they stimulate the tissues to carry out specific functions. Progesterone is essential for the proper functioning of the female menstrual cycle and formation of the placenta during pregnancy.

The chemical name for progesterone is pregn-4-ene-3,20-dioneand its symbol is C₂₁H₃₀O₂. The main site of production is in the cells of the corpus luteum of the ovaries. There are two types of cells in the ovaries that produce progesterone. In one type of cell, the small luteal cells, luteinizing hormone (LH) stimulates the secretion of progesterone. Large luteal cells also produce progesterone, but their production is controlled by a chemical called prostaglandin (PG)F2alpha which causes the death of these particular cells.

The production of progesterone is limited by the synthesis of cholesterol. The mitochondria of cells transport cholesterol across their membranes (steroidogenesis) and the rate at which this happens is regulated by the speed at which cholesterol is transported across the inner and outer mitochondrial membrane. The initiation of progesterone production begins in this area of the mitochondrion.

The effect of progesterone is most keenly observed in the uterus where the hormone aids in the successful implantation of a fertilized egg. It stimulates the growth of the placenta where additional progesterone is produced. In this case progesterone prevents spontaneous abortion. The hormone also targets tissues of the mammary glands and prepares them to secrete milk. If no fertilization occurs, the synthesis of progesterone is reduced and the onset of menstruation occurs. Progesterone is also produced in the adrenal cortex where it is used for the synthesis of testosterone and other steroids.

Today progesterone is synthesized in laboratories and is a useful medication. Some ovarian disorders are regulated by the consumption of synthetic progesterone. It has been successfully used in combination with other hormones for birth control.