Menstruation

Menstruation, in women of reproductive age, specifically the monthly loss of blood (period), representing shedding of womb endometrium; in general, the whole monthly cycle of hormonal, structural, and functional changes, punctuated by menstrual blood loss. After each period, the endometrium (womb-lining) starts to proliferate and thicken under the influence of gonadotrophins (follicle-stimulating hormone) and estrogens. In midcycle an egg is released from an ovarian follicle (ovulation). The endometrium is prepared for implantation of a fertilized egg. If the egg is not fertilized, pregnancy does not ensue; then blood-vessel changes that occur lead to the shedding of the endometrium and some blood, sometimes with pain or colic. The cycle then restarts. Cyclic patterns are established at puberty (menarche) and end in middle life (age 45–60) at the menopause, the “change of life.” Disorders of menstruation include heavy, irregular, or missed periods; bleeding between periods or after the menopause; and excessively painful periods. These disorders are studied in gynecology.

See also: Reproduction.