Uterine contraction

The uterus frequently contracts throughout the entire menstrual cycle, and these contractions have been termed endometrial waves or contractile waves.[1] These appear to involve only the sub-endometrial layer of the myometrium.[1] 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.[1] 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.[1] These contractions are sometimes termed menstrual cramps,[2] 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.[1]

Knitted Bellyband with conductive thread and RFID chip to monitor contractions

A contraction refers specifically to the motion of the uterus[3] 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 cardiotocography, in which a device is fixated to the skin of the mother or directly to the fetal scalp. The pressure required to flatten a section of the uterine wall correlates with the internal pressure, thereby providing an estimate of it.[4]

A type of monitoring technology under development at Drexel University embeds conductive threads in the knitted fabric of a bellyband. When the fibers stretch in response to a contraction, the threads function like an antenna, and send the signals they pick up to an embedded RFID (radio frequency identification device) chip that reports the data.[5]

The uterus and vagina contract during female orgasm.[6] These contractions may not be noticed by all women; pregnant women are more likely to notice these contractions by late 2nd and 3rd trimesters.

• Involution (medicine)