progesterone, female sex hormone that induces secretory changes in the lining of the uterus essential for successful implantation of a fertilized egg. A steroid, progesterone is secreted chiefly by the corpus luteum, a group of cells formed in the ovary after the follicle ruptures during the release of the egg cell. If fertilization does not take place, the secretion of progesterone decreases and menstruation occurs. If fertilization does occur, progesterone is secreted during pregnancy by the placenta and acts to prevent spontaneous abortion; the hormone also prepares the mammary glands for milk production. Progesterone is also synthesized from cholesterol in the cortex of the adrenal gland where it is a precursor for the synthesis of other steroids including testosterone. Synthetic compounds with progesteronelike activity have been developed that, along with estrogen, are used in oral contraceptives.

Steroid hormone secreted by the female reproductive system that functions mainly to regulate the condition of the endometrium (see uterus), preparing it to accept a fertilized egg. If the egg is not fertilized, the level of progesterone drops, the uterine lining breaks down, and menstruation ensues. If the egg is fertilized (see pregnancy), the placenta produces progesterone, whose effects include preparing the mammary glands for lactation. Many forms of oral contraception use a synthetic progesterone.

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