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It follows that some process of adaptation or accommodation of suitable portions for the head to the various pelvic planes is necessary to insure the completion of childbirth. This is brought about by certain movement of the presenting part, which belong to what is termed the mechanism of labour.

—J. Whitridge Williams (1903)

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INTRODUCTION

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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. Pregnancy and birth are physiological processes, and thus, labor and delivery should be considered normal for most women.

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MECHANISMS OF LABOR

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Pelvic Floor Changes

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Many adaptive changes are required for pregnancy and for labor and delivery. According to Nygaard (2015), vaginal delivery is a traumatic event. To assess this in part, Staer-Jensen and colleagues (2015) obtained transperineal sonographic measurements of the pelvic floor muscles at 21 weeks’ and 37 weeks’ gestation, and again at 6 weeks, 6 months, and 12 months postpartum. In 300 nulliparas, they measured bladder neck mobility and the area within the urogenital hiatus during Valsalva. This hiatus is the U-shaped opening in the pelvic floor muscles through which the urethra, vagina, and rectum pass (Chap. 2, Perineum). In this study, the levator hiatus area was significantly larger at 37 weeks’ gestation and at 6 weeks postpartum compared with earlier pregnancy. Then, by 6 months postpartum, the hiatus had improved and narrowed to return to an area comparable to that at 21 weeks’ gestation. However, no further improvement was noted by 12 months postpartum. Of note, hiatal area enlargement was only seen in those who delivered vaginally.

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These findings demonstrate antepartum changes in pelvic floor structure that may reflect adaptations needed to permit vaginal delivery (Nygaard, 2015). Additional pelvic floor changes are discussed in Chapter 4 (Fallopian Tubes), and the contributions of pregnancy and delivery to later pelvic organ prolapse and incontinence are described in Chapter 30 (Cesarean Delivery Risks).

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Fetal Lie

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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.

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Fetal lie describes the relationship of the fetal long axis to that of the mother. In more than 99 percent of labors at term, the fetal lie is longitudinal. A transverse lie is less frequent, and predisposing factors include multiparity, placenta previa, hydramnios, and uterine anomalies (Chap. 23, Brow Presentation). Occasionally, the fetal and maternal axes may cross at a 45-degree angle, forming an oblique lie. This is unstable and becomes longitudinal or transverse during labor.

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Fetal Presentation

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