Labor is defined as repeated uterine contractions that result in the dilation and effacement of the uterine cervix causing the fetus to be expelled. The mechanisms that initiate labor are not completely known. However, it is clear that preparation for labor begins long before with changes in the cervix that include ingrowth of C-type pain fibers1,2 and infiltration of inflammatory cells, including neutrophils and macrophages.3 These inflammatory cells release cytokines that result in softening of the cervix in preparation for labor.
Corticotropin-releasing hormone (CRH) may play a role in the initiation of labor. As the placenta matures, it produces increasing maternal levels of CRH, which peak at the time of delivery. The increase in CRH contributes to a feed-forward cycle in which increasing concentrations of CRH stimulate the pituitary to produce corticotropin, which induces cortisol production in the adrenal glands of both mother and fetus. Cortisol then enhances expression of the CRH gene, resulting in increased expression of CRH.4 Indeed, elaboration of CRH at an unusually high rate is an independent predictor of preterm birth.5
Uterine muscle or myometrium is a specialized type of smooth muscle that exhibits a positive feedback loop in response to pressure on the uterine cervix. Activation of sensory neurons in the cervix enhances the release of oxytocin from the posterior pituitary, which increases contractility. Increasing estrogen and progesterone during pregnancy cause an up-regulation of oxytocin receptors in uterine muscle. At term, there is functional progesterone withdrawal that is mediated by increasing expression of type A progesterone receptor relative to the type B progesterone receptor, which suppresses progesterone responsiveness.6
Labor has classically been separated into three stages. The first stage of labor includes the change of the uterine cervix from a thick closed tube approximately 3 cm long to an opening of approximately 10 cm through which the fetus can be expelled. The second stage of labor is the expulsion of the fetus through that opening. The third stage is the expulsion of the placenta. The time course of the first stage of labor was first studied by Emanuel Friedman, who described the course of labor as sigmoidal (Figure 3-1).7 Friedman separated the first stage of labor into a latent phase, an active phase, and a deceleration phase. The sigmoidal nature of the relationship has since been challenged; there is little evidence for a deceleration phase as the cervix approaches complete dilation (10 cm).8,9 However, the separation of the first stage of labor into an early slow phase, called latent labor, and a more rapid phase of active labor has withstood the test of time.8,9
Representative tracing of the course of cervical dilation in the labor of a nulliparous patient. Estimates of cervical dilation (vertical axis) have been plotted against elapsed time in labor (horizontal axis). ...