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Labor is the process that leads to childbirth. It begins with the onset of regular uterine contractions and ends with delivery of the newborn and expulsion of the placenta. The term labor in the obstetrical context takes on several connotations from the English language. It is undoubtedly true that pregnancy and birth are physiological processes, and as such, labor and delivery should be considered to be normal for most women (Lawrence, 2012). This understanding of normal labor and delivery as a physiological process has come under some scrutiny in the past decade because pelvic floor disorders have been observed to be more prevalent among women who have delivered at least one child (Handa, 2011; Nygaard, 2008). Determining which aspects of childbirth contribute most to this risk has become an area of intense investigation and discussed further in Chapter 30 (Cesarean Delivery Indications and Risks).

Mechanisms of Labor

At the onset of labor, the position of the fetus with respect to the birth canal is critical to the route of delivery and thus should be determined in early labor. Important relationships include fetal lie, presentation, attitude, and position.

Fetal Lie

The relation of the fetal long axis to that of the mother is termed fetal lie and is either longitudinal or transverse. Occasionally, the fetal and the maternal axes may cross at a 45-degree angle, forming an oblique lie. This lie is unstable and becomes longitudinal or transverse during labor. A longitudinal lie is present in more than 99 percent of labors at term. Predisposing factors for transverse fetal position include multiparity, placenta previa, hydramnios, and uterine anomalies (Chap. 23, Etiology and Diagnosis).

Fetal Presentation

The presenting part is that portion of the fetal body that is either foremost within the birth canal or in closest proximity to it. It typically can be felt through the cervix on vaginal examination. Accordingly, in longitudinal lies, the presenting part is either the fetal head or breech, creating cephalic and breech presentations, respectively. When the fetus lies with the long axis transversely, the shoulder is the presenting part. Table 22-1 describes the incidences of the various fetal presentations.

TABLE 22-1Fetal Presentation in 68,097 Singleton Pregnancies at Parkland Hospital
Cephalic Presentation

Such presentations are classified according to the relationship between the head and body of the fetus (Fig. 22-1). Ordinarily, the head is flexed sharply so that the chin is in contact with ...

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