menstrual cycle

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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 menopause. The stages of the menstrual cycle are shown in the diagram. 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.

The menstrual cycle.

Subjects: Medicine and Health.

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