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. Thus, labor and delivery should be considered normal for most women.
Fetal position within the birth canal is critical to labor progress and to the delivery route. It should be determined in early labor, and sonography can be implemented for unclear cases. Important relationships include fetal lie, presentation, attitude, and position.
Of these, 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. Occasionally, the fetal and maternal axes may cross at a 45-degree angle to form an oblique lie. This is unstable and becomes longitudinal or transverse during labor.
The presenting part is the portion of the fetal body either within or in closest proximity to the birth canal. It usually can be felt through the cervix on vaginal examination. 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 considered the presenting part.
Cephalic presentations are subclassified 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 contacts the thorax. 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—face presentation. The fetal head may assume a position between these extremes. When the neck is only partly flexed, the anterior (large) fontanel may present—sinciput 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 occiput or face presentations by neck flexion or extension, respectively. If not, dystocia can develop (Chap. 23, p. 441).
Longitudinal lie, cephalic presentation. Differences in attitude of the fetal body in (A) occiput, (B) sinciput, (C) brow, and (D) face presentations. Note changes in fetal attitude as the fetal head becomes less flexed.
The fetus at term usually presents occiput rather than breech. Although the fetal head at term is slightly larger than the breech, the entire podalic pole of the fetus—that is, the ...