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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.
The primary function of uterine contractions is to expel the fetus from the uterine cavity. However, contractions also play an essential role in minimizing postpartum hemorrhage.
Oxytocin is a natural hormone that stimulates uterine contractions in childbirth and lactation after childbirth. It also affects aspects of human behavior and the male and female reproductive systems.
A uterus contraction starts in the fundus and sweeps down to the rest of the uterus. Uterine muscles shorten in response to the contraction, then relax, leading to fetal descent. Uterine contractions are measured in mmHg.
Uterine contractions become expulsive and this pushes the foetus through the birth canal. There are two stages of this: The passive stage – this lasts until the head of the foetus reaches the pelvic floor, at which point the woman experiences the desire to push.
Labor contractions are the periodic tightening and relaxing of the uterine muscle, the largest muscle in a woman's body. Something triggers the pituitary gland to release a hormone called oxytocin that stimulates the uterine tightening. It is difficult to predict when true labor contractions will begin.
When oxytocin is released, it stimulates uterine contractions, and these uterine contractions, in turn, cause more oxytocin to be released; this is what causes the increase in both the intensity and frequency of contractions and enables a mother to carry out vaginal delivery completely.
As labor nears, oxytocin begins to stimulate stronger, more painful uterine contractions, which—in a positive feedback loop—stimulate the secretion of prostaglandins from fetal membranes. Like oxytocin, prostaglandins also enhance uterine contractile strength.
Uterine contractions (UC) are characterized by three parameters: frequency, amplitude, and direction of contraction. Also pertinent to the contractile status of the uterus is the IUP value that prevails between UCs or resting tone.
We will briefly overview growth of the uterus in pregnancy, describe the uterine metabolites involved in contraction and then discuss metabolism and relation between contractions and blood flow. This latter part will include recent data examining the effects of hypoxia and pH in the myometrium.