A uterine contraction is a muscle contraction of the uterine smooth muscle.
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Throughout menstrual cycle
The uterus frequently contracts throughout the entire menstrual cycle, and these contractions have been termed endometrial waves or contractile waves. These appear to involve only the sub-endometrial layer of the myometrium. In the early follicular phase, these contractions occur once or twice per minute and last 10–15 seconds with a low amplitude of usually 30 mmHg. The frequency increases to 3–4 per minute towards ovulation. During the luteal phase, the frequency and amplitude decrease, possibly to facilitate any implantation.
If implantation does not occur, the frequency remains low, but the amplitude increases dramatically to between 50 and 200 mmHg producing labor-like contractions at the time of menstruation. These contractions are sometimes termed menstrual cramps, although that term is often used for menstrual pain in general. These contractions may be uncomfortable or even painful, but they are generally significantly less painful than contractions during labour. A hot water bottle or exercising has been found to help.
A shift in the myosin expression of the uterine smooth muscle has been hypothesized to avail for changes in the directions of uterine contractions that are seen during the menstrual cycle.
Childbirth
A contraction refers specifically to the motion of the uterus as part of the process of childbirth. Contractions, and labour in general, is one condition that releases the hormone oxytocin into the body. Contractions become longer as labour intensifies.
Prior to actual labour, women may experience Braxton Hicks contractions, sometimes known as "false labour."
Since every pregnancy is different, a doctor, midwife or other competent professional should always be consulted before any action is taken to reduce the pain. Some popular methods may be harmful to the mother and/or the baby, or may actually worsen the pain or lengthen the labour.
Uterine contractions during childbirth can be monitored by tocography, in which a device called a tocodynamometer, which has a flat area that is fixated to the skin by a band around the belly. The pressure required to flatten a section of the wall correlates with the internal pressure, thereby providing an estimate of it.
In orgasm
The uterus and vagina contract during female orgasm. These contractions may not be noticed by all women; pregnant women are more likely to notice these contractions by late 2nd and 3rd trimesters.