Menstrual cycle

The monthly cycle of changes in the ovaries and the lining of the uterus (endometrium), starting with the preparation of an egg for fertilization. When the follicle of the prepared egg in the ovary breaks, it is released for fertilization and ovulation occurs. Unless pregnancy occurs, the cycle ends with the shedding of part of the endometrium, which is menstruation. Although it is actually the end of the physical cycle, the first day of menstrual bleeding is designated as "day 1" of the menstrual cycle in medical parlance.

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menstrual cycle n the whole cycle of physiological changes from the beginning of one menstrual period to the beginning of the next

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the periodic sequence of events in sexually mature nonpregnant women by which an egg cell (ovum) is released from the ovary at four-weekly intervals until the change of life (see menopause). An ovum develops within a Graafian follicle in the ovary. When mature, it bursts from the follicle and travels along the Fallopian tube to the uterus. A temporary endocrine gland - the corpus luteum - develops in the ruptured follicle and secretes the hormone progesterone, which causes the lining of the uterus (endometrium) to become thicker and richly supplied with blood in preparation for pregnancy. If the ovum is not fertilized the cycle continues: the corpus luteum shrinks and the endometrium is shed at menstruation. If fertilization does take place the fertilized ovum becomes attached to the endometrium and the corpus luteum continues to secrete progesterone, i.e. pregnancy begins.

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the type of sexual cycle seen in female humans and some other primates, with physiologic changes in the endometrium that recur at regular intervals during the reproductive years; it is subdivided into two cycles, the ovarian and uterine cycles, according to the organ under consideration. By convention, the menstrual cycle begins with the first day of menstruation, followed by the follicular and luteal phases in the ovary and the corresponding proliferative and secretory phases in the uterus. If pregnancy occurs, the menstrual cycles cease, otherwise the reduction in estrogen and progesterone production leads to constriction of the spiral arteries and endometrial ischemia, culminating in menstruation (partial shedding of the endometrium with bleeding from the vagina). Cf. estrous c. See illustration.

Changes in the menstrual cycle in the human female. Solid lines indicate the course of events when the oocyte is not fertilized; dotted lines indicate the course of events when fertilization occurs. Arrows indicate the actions of hormones of the pituitary and the ovary in regulating the cycle.