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.

This entry comes from Encyclopædia Britannica Concise.
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