Uterine contraction

Uterine contractions are muscle contractions of the uterine smooth muscle that can occur at various intensities in both the non-pregnant and pregnant uterine state. The non-pregnant uterus undergoes small, spontaneous contractions in addition to stronger, coordinated contractions during the menstrual cycle and orgasm. Throughout gestation, the uterus enters a state of uterine quiescence due to various neural and hormonal changes. During this state, the uterus undergoes little to no contractions, though spontaneous contractions still occur for the uterine myocyte cells to experience hypertrophy. The pregnant uterus only contracts strongly during orgasms, labour, and in the postpartum stage to return to its natural size.

Uterine contractions that occur throughout the menstrual cycle, also termed endometrial waves or contractile waves, appear to involve only the sub-endometrial layer of the myometrium.

In the early follicular phase, uterine contractions in the non-pregnant woman occur 1–2 times per minute and last 10–15 seconds with a low intensity of usually 30 mmHg or less. This sub-endometrial layer is rich in estrogen and progesterone receptors. The frequency of contractions increases to 3–4 per minute towards ovulation. During the luteal phase, the frequency and intensity decrease, possibly to facilitate any implantation.

If implantation does not occur, the frequency of contractions remains low; but at menstruation the intensity increases dramatically to between 50 and 200 mmHg producing labor-like contractions. These contractions are sometimes termed menstrual cramps, although that term is also 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. Painful contractions are called dysmenorrhea.

A shift in the myosin expression of the uterine smooth muscle has been hypothesized as arising for changes in the directions of uterine contractions during the menstrual cycle.

Uterine contractions are a vital part of natural childbirth, which occur during the process of labour and delivery, (typically this excludes caesarean section). These labour contractions are characterized by their rhythmic tightening and relaxation of the myometrium, the most prominent uterine muscle. Labour contractions primarily serve the purpose of opening and dilating the cervix, which leads to the assisting of the passage of the baby through the vaginal canal during the first stage of labour.

Throughout pregnancy, the uterus experiences motor denervation, thus inhibiting spontaneous contractions. The remaining contractions are predominantly hormonally controlled. The decrease in the coordination of uterine smooth muscles cells reduces the effectiveness of contractions, causing the uterus to enter a state of uterine quiescence. During the beginning of labour, contractions may initially be intermittent and irregular, but will transition into a more coordinated pattern as the labour progresses. This transition is governed by various myogenic, neurogenic, and hormonal factors working together. As labour progresses, contractions will typically increase in frequency and intensity, which leads to a significant rise in intrauterine pressure.

Otherwise, not all contractions experienced by pregnant individuals are indications of the beginning of labour. Some women experience what are commonly called Braxton Hicks contractions before their initial due date, which are characterized as “false labour." Though similar to labour uterine contractions, these contractions do not play a prominent role in cervical dilation or the progression of childbirth.