Menstruation is the shedding of the uterine lining (endometrium). It occurs on a regular basis in reproductive-age females of certain mammal species.
Overt menstruation (where there is bleeding from the uterus through the vagina) is found primarily in humans and close evolutionary relatives such as chimpanzees.
The females of other placental mammal species have estrous cycles, in which the endometrium is reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle. Many zoologists regard this as different from a "true" menstrual cycle.
Menstruation lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7
days is considered normal). The average blood loss during menstruation is 35 millilitres with 10-80 mL considered normal; many women also notice shedding of the endometrium lining
that appears as tissue mixed with the blood. (Sometimes this is
erroneously thought to indicate an early-term miscarriage of an
An enzyme called plasmin — contained in the endometrium —
tends to inhibit the blood from clotting. Because of this blood loss,
premenopausal women have higher dietary requirements for iron to
prevent iron deficiency.
Many women experience uterine cramps, also
referred to as dysmenorrhea, during this time, caused largely by the
contractions of the uterine muscle as it expels the endometrial blood
from the woman's body. A vast industry has grown to provide drugs to
aid in these cramps, as well as sanitary products to help manage menses.
Menstruation is the most visible phase of the menstrual
cycle. Menstrual cycles are counted from the first day of menstrual
bleeding, because the onset of menstruation corresponds closely with
the hormonal cycle.
During pregnancy and for some time after childbirth,
menstruation is normally suspended; this state is known as amenorrhoea,
i.e. absence of the menstrual cycle.
If menstruation has not resumed,
fertility is low during lactation. The average length of postpartum
amenorrhoea is longer when certain breastfeeding practices are
followed; this may be done intentionally as birth control (lactational
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Last Updated: Feb 1, 2011