Menstrual cycle.
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. Naturally occurring hormones drive two concurrent and coordinated cycles that last about 28 days. The ovarian cycle controls the production and release of eggs and the release of the hormones estrogen and progesterone. The uterine cycle governs the preparation and maintenance of the lining of the uterus (endometrium) to receive a fertilized egg. Menstruation, a "period" in common parlance, begins on day one of the cycle, and lasts for about five days. Around day fourteen, an egg is usually released from the ovary. Estrogen stimulates the uterus lining to thicken to accommodate an embryo should fertilization occur. If implantation does not occur, the lining is released during menstruation. Menarche (the first period) usually occurs around age twelve, and menstrual cycles usually continue for 30–45 years until menopause.
