Reproductive activity in the female normally begins during puberty with menarche, the first menstrual period. Each month, the menstrual cycle is controlled, like reproductive activity in the male, by hormones from the anterior pituitary gland. Follicle stimulating hormone (FSH) begins the cycle by causing the ovum to ripen in the graafian follicle. The follicle secretes estrogen, a hormone that starts development of the endometrium in preparation for the fertilized egg. A second pituitary hormone, luteinizing hormone (LH), triggers ovulation and conversion of the follicle to the corpus luteum. This structure, left behind in the ovary, secretes progesterone and estrogen, which further the growth of the endometrium. If no fertilization occurs, hormone levels decline, and the endometrium sloughs off in the process of menstruation.
The average menstrual cycle lasts 28 days, with the first day of menstruation taken as day 1 and ovulation occurring on about day 14. Throughout the cycle, estrogen and progesterone feed back to the pituitary to regulate the production of FSH and LH. Hormonal methods of birth control act by supplying estrogen and progesterone, which inhibit the pituitary and prevent ovulation, while not interfering with menstruation.
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