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 pregnant uterus only contracts strongly during orgasms, labour, and in the postpartum stage to return to its natural size.

Throughout menstrual cycle

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. 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.

Directionality of contractions

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. 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, During the beginning of labour, contractions may initially be intermittent and irregular, Oxytocin is produced by the body naturally and since the 1950s has also been available in synthetic pharmaceutical form. In either form, oxytocin stimulates uterine contractions to accelerate the process of childbirth. Production and secretion of oxytocin is controlled by a positive feedback mechanism, where its initial release, either naturally or in pharmaceutical form, stimulates production and release of further oxytocin. For example, when oxytocin is released during a contraction of the uterus at the start of childbirth, this stimulates production and release of more oxytocin and an increase in the duration, intensity and frequency of contractions. This process compounds in intensity and frequency and continues until the triggering activity ceases.

Prostaglandins

The concentration of prostaglandins in the blood plasma and amniotic fluid increases during labor.

Monitors

thumb|right | Knitted Bellyband with conductive thread and RFID chip to monitor contractions

Uterine contractions can be monitored by cardiotocography, in which a device is affixed 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.

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.

Mechanism

Resting state

The resting membrane potential (V<sub>rest</sub>) of uterine smooth muscle has been recorded to be between −35 and −80 mV.

Restoration to resting state

Uterine smooth muscle mechanisms of relaxation differ significantly from those of other human smooth muscles. And while the peptide hormone relaxin has been shown to inhibit uterine contractility in rats, mice, and pigs, it does not prevent uterine contractility in humans.