menstrual

menstrual cycle the regularly recurring physiologic changes in the endometrium that culminate in its shedding (menstruation). Menstrual cycles vary in length, with the average being about 28 days. The length of time of menstrual flow is also variable, with an average of about 5 days. Women menstruate from puberty to menopause, except during pregnancy. The first 14 days of the cycle are called the follicular phase; a follicle containing an ovum is developing in one of the ovaries. It begins as the menstrual flow ceases; the lining of the uterus is stimulated by estrogen and begins to increase in thickness to prepare for the possibility of reproduction. On the twelfth or thirteenth day of the cycle, the ovulatory phase begins with a surge in levels of luteinizing hormone and follicle-stimulating hormone; ovulation then takes place and the ovary discharges the ovum. The ruptured follicle is transformed into a yellowish material called the corpus luteum; the luteal phase begins as the corpus luteum secretes progesterone. Progesterone acts on the endometrium, building up tissues with an enriched supply of blood to nourish the future embryo. If fertilization and conception do not take place, the estrogen level in the blood falls, the endometrium is no longer stimulated, and the uterus again becomes thinner. Blood circulation slows, blood vessels contract, and the menstrual phase begins; unused tissue breaks down into the bloody discharge known as menstruation. The cycle then starts again.