Library · Physiology
Labor
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Labor is the normal process by which the body moves the fetus and the placenta out of the uterus, through the vagina. It is driven by regular contractions of the uterus, which gradually open (dilate) and thin out (efface) the cervix, the outlet of the uterus. Think of the neck of a turtleneck sweater when you pull it over your head: it stretches, thins, and opens.
In the first stage, one hormone is key: oxytocin. The posterior pituitary, a gland beneath the brain, releases it into the blood, and the uterus contracts. The fetus's head then moves down and stretches the cervix, and that stretching triggers the release of more oxytocin. This is called positive feedback: the loop feeds itself, and contractions and dilation keep intensifying.
The whole process is divided into three stages. The first ends when the cervix reaches full dilation, 10 centimeters. The second ends with the birth. And the third, with the delivery of the placenta.
In brief
Labor is the normal process by which the fetus and placenta leave the uterus through the vagina; regular contractions dilate and efface the cervix.
In the first stage, oxytocin from the posterior pituitary is key: it contracts the uterus, the fetal head descends and stretches the cervix, and that releases more oxytocin. This positive feedback intensifies contractions and dilation.
The first stage ends at full dilation of 10 cm; the second, with birth; the third, when the placenta is delivered.
Sources (2)
- National Center for Biotechnology Information, StatPearls PublishingNormal Labor: Physiology, Evaluation, and Management, StatPearlsPublished
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Cervical Dilation, StatPearlsPublished
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