These indings demonstrate antepartum changes in pelvic loor structure that may relect adaptations needed to permit vaginal delivery (Nygaard, 2015). Additional pelvic loor changes are discussed in Chapter 4 (p. 52), and the contributions of pregnancy and delivery to later pelvic organ prolapse and incontinence are described in Chapter 30 (p. 568).
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.
TABLE 22-1 . Fetal Presentation in 68,097 Singleton Pregnancies at Parkland Hospital
Cephalic 96.8 Breech 2.7 1s:36
Transverse lie 0.3 1 :335
Compound 0.1 1:1000
Face 0.05 1 :2000 Brow 0.01 1 :10,000
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, p. 452). Occasionally, the fetal and maternal axes may cross at a 45-degree angle, forming an oblique lie. his is unstable and becomes longitudinal or transverse during labor.
The presenting part is the 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 during vaginal examination. Accordingly, in longitudinal lies, the presenting part is either the fetal head or the 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 these various presentations.
These presentations are classified according to the relationship between the head and body of the fetus (Fig. 22-1). Ordinarily, the head is lexed sharply so that the chin contacts the thorx. The occipital fontanel is the presenting part, and this presentation is referred to as a vertex or occiput presentation. Much less often, the fetal neck may be sharply extended so that the occiput and back come into contact, and the face is foremost in the birth canal-oce presentation. The fetal head may assume a position between these extremes. When the neck is only partly lexed, the anterior (large) fontanel may present-sincput presentation. When the neck is only partially extended, the brow may emerge-brow presentation. These latter two are usually transient. As labor progresses, sinciput and brow presentations almost always convert into vertex or face presentations by neck lexion or extension, respectively. Failure to do so can lead to dystocia, discussed in Chapter 23 (p. 452).
FIGURE 22-1 Longitudinal lie. Cephalic presentation. Differences in attitude of the fetal body in (A) vertex, (B) sinciput, (C) brow, and (0) face presentations. Note changes in fetal attitude in relation to fetal vertex as the fetal head becomes less flexed.