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LABOR The physiologic process by which the products of conception (fetus, amniotic fluid, placenta, and membranes) are separated and expelled from the uterus through the vagina into the outside world. It is defined by the presence of regular uterine contractions accompanied by cervical effacement and dilatation and fetal descent.

PRETERM LABOR The onset of labor in a gravid woman whose period of gestation is less than 37 completed weeks (less than 259 days) from the first day of the last menstrual period.

DYSTOCIA Abnormally slow and protracted labor.

EXPECTED DATE OF CONFINEMENT On an average, this is 280 days from the first day of the last menstrual period or 267 days from conception. It is calculated by going back 3 months from the first day of the last menses and adding 7 days (Naegle's rule)


Causes of Onset of Labor

Hippocrates' concept that a fetus determines the time of his or her birth has been proven correct in some animals. In humans, however, it appears that the placenta and fetal membranes play the major role in the initiation of labor, while the fetus may modulate the timing of labor. Although the exact cause of and mechanism for labor are not known, evidence is mounting in support of a hormonal basis.

In sheep, it is clear that maturation of the fetal hypothalamo–hypophyseal–adrenal axis during late pregnancy is responsible for initiating labor by inducing changes in the pattern of placental steroid genesis and, ultimately, by increasing the production of intrauterine prostaglandin. Birth can be induced by infusing adrenocorticotropic hormone (ACTH) or glucocorticoids to the fetal lamb in utero before term has been reached. These preterm fetuses are viable and are able to expand their lungs, indicating that the fetal glucocorticoids play a role in pulmonary maturation as well as in parturition. In sheep as in humans, the nature of the stimulus that leads to increased pituitary–adrenal activity in late pregnancy is not known.

At the present time, there is no evidence that human fetuses play the same pivotal role in determining the time they are born as do sheep. The belief that anencephaly and adrenal hypoplasia predispose to prolongation of pregnancy has been challenged. In both humans and monkeys, the administration of glucocorticoids does not bring on labor, nor is there evidence to show that fetal cortisol sets off parturition in humans.


Although some evidence shows that estrogen is involved in human parturition, its mode of action has not been well defined. The placenta is the main source of estrogen biosynthesis in pregnancy. Estrogens upregulate myometrial gap junctions and receptors responsible for myometrial contractions (e.g., calcium channels and oxytocin receptors).


In some animals, progesterone plays a part in maintaining uterine quiescence. In ...

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