Menstrual Cramps (cont.)
What causes menstrual cramps?
Each month, the inner lining of the uterus (the endometrium) builds up in preparation for a possible pregnancy. After ovulation, if the egg is not fertilized by a sperm, no pregnancy will result and the current lining of the uterus is no longer needed. The woman's estrogen and progesterone hormone levels decline, and the lining of the uterus becomes swollen and dies. It is then shed and will be replaced by a new lining in the next monthly cycle.
When the old uterine lining begins to break down, molecular compounds called prostaglandins are released. These compounds cause the muscles of the uterus to contract. When the uterine muscles contract, they constrict the blood supply (vasoconstriction) to the endometrium. This contraction blocks the delivery of oxygen to the tissue of the endometrium which, in turn, breaks down and dies. After the death of this tissue, the uterine contractions literally squeeze the old endometrial tissue through the cervix and out of the body by way of the vagina. Other substances known as leukotrienes, which are chemicals that play a role in the inflammatory response, are also elevated at this time and may be related to the development of menstrual cramps.
Why are some cramps so painful?
Menstrual cramps are caused by the uterine contractions that occur in response to prostaglandins and other chemicals. The cramping sensation is intensified when clots or pieces of bloody tissue from the lining of the uterus pass through the cervix, especially if a woman's cervical canal is narrow.
The difference between menstrual cramps that are more painful and those that are less painful may be related to a woman's prostaglandin levels. Women with menstrual cramps have elevated levels of prostaglandins in the endometrium (uterine lining) when compared with women who do not experience cramps. Menstrual cramps are very similar to those a pregnant woman experiences when she is given prostaglandin as a medication to induce labor.
Next: Can menstrual cramps be measured? »
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