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.
Read more about this topic: Uterine Contraction
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