Oxytocin and the initiation of parturition. A review.

Oxytocin (OT) binds to specific receptors of myometrial cells, inducing and increasing myometrial contractions. During pregnancy and especially close to term, an increase in the myometrial OT-receptor concentration is found, leading to an increased sensitivity of the myometrium towards circulating OT. The factors determining the receptor level are not completely understood, but may include the level of steroids, OT- and oestrogenreceptors. Because of the increased sensitivity, only a small increase in the maternal OT blood-level is necessary to induce myometrial contractions at term. The level of maternal plasma OT does not change significantly throughout pregnancy. The fetus is found to secrete considerable amounts of OT during the first stage of labour, which reaches the myometrium in spite of the high level of oxytocinase in placenta. At the second stage of labour the distension of the lower birth canal might cause release of OT from the maternal neurohypophysis into the blood, increasing the myometrial contractions. This mechanism is observed in animals, but not established in the human.