The term fetus usually presents with the vertex, most logically because the uterus is piriform or pear shaped. Although the fetal head at term is slightly larger than the breech, the entire podalic pole of the fetus-that is, the breech and extremities-is bulkier and more mobile than the cephalic pole. The cephalic pole is composed of the fetal head only. Until approximately 32 weeks, the amnionic cavity is large compared with the fetal mass, and the fetus is not crowded by the uterine walls. Subsequently, however, the ratio of amnionic luid volume declines relative to the growing fetal mass. As a result, the uterine walls are apposed more closely to the fetal parts. The fetus orients its polarity to make use of the roomier fundus for its bulkier and more mobile podalic pole. The high incidence of breech presentation in hydrocephalic fetuses is in accord with this theory, as the larger fetal cephalic pole requires more room than its podalic pole.
The incidence of breech presentation drops with gestational age and approximates 3 percent at term. When the fetus presents breech, the three general configurations arerank, complete, and ootling presentations, described in Chapter 28 (p. 539). Breech presentation may result from circumstances that prevent normal version from taking place. One example is a septum that protrudes into the uterine cavity (Chap. 3, p. 45). Variances of fetal attitude, particularly extension of the vertebral column as seen in frank breeches, also may prevent the fetus from turning. If the placenta is implanted in the lower uterine segment, it may distort normal intrauterine anatomy and result in a breech presentation.
In the later months of pregnancy, the fetus assumes a characteristic posture described as attitude or habitus (see Fig. 22-1). As a rule, the fetus forms an ovoid mass that corresponds roughly to the shape of the uterine cavity. The fetus becomes folded upon itself to create a convex back. The head is sharply lexed; the chin is almost in contact with the chest; the thighs are lexed over the abdomen; and the legs are bent at the knees. In all cephalic presentations, the arms usually lie across the thorax or parallel to the sides. The umbilical cord ills the space between the
FIGURE 22-2 Longitudinal lie. Vertex presentation. A. Left occiput anterior (LOA). B. Left occiput posterior (LOP).
extremities. his characteristic posture results from the mode of fetal growth and its accommodation to the uterine cavity.
Abnormal exceptions to this attitude occur as the fetal head becomes progressively more extended from the vertex to the face presentation. This results in a progressive change in fetal attitude from a convex (flexed) to a concave (extended) contour of the vertebral column.
Position refers to the relationship of an arbitrarily chosen portion of the fetal presenting part to the right or left side of the birth canal. Accordingly, with each presentation, there may be two positions-right or left. he fetal occiput, chin (mentum), and sacrum are the determining points in vertex, face, and breech presentations, respectively (hgs. 22 2 to
Because the presenting part may be in either the left or right position, there are left and right occipital (LO and RO), left and right mental (LM and RM), and left and right sacral (LS and RS) designations.
Further, the relationship of a given portion of the presenting part to the anterior (A), transverse (T), or posterior (P) portion of the maternal pelvis is considered. As shown in Figures 22-2 to 22-6, there are six varieties of each of the three presentations. Thus, in an occiput presentation, the presentation, position, and variety may be abbreviated in clockwise fashion as:
Approximately two thirds of all vertex presentations are in the let occiput position, and one third in the right.