![]() The middle letter stands for the specific presenting part of the fetus (O for occiput, S for sacrum, M for mentum, and Sc for scapula ). The first letter of the abbreviation denotes the location of the presenting part in the right (R) or left (L) side of the mother’s pelvis. Position is denoted by a three-letter abbreviation. Position is the relationship of a reference point on the presenting part (occiput, sacrum, mentum or sinciput ) to the four quadrants of the mother’s pelvis (see Fig. The presentation or presenting part indicates the portion of the fetus that overlies the pelvic inlet. As the head is more extended, the anteroposterior diameter widens, and the head may not be able to enter the true pelvis (see Fig. When the fetal head is in complete flexion, this diameter allows it to pass through the true pelvis easily (see Fig. Of the several anteroposterior diameters, the smallest and most critical one is the suboccipitobregmatic diameter (about 9.5 cm at term). In a well-flexed cephalic presentation the biparietal diameter is the widest part of the head entering the pelvic inlet. The biparietal diameter, which is about 9.25 cm at term, is the largest transverse diameter and an important indicator of fetal head size ( Fig. For example, in a cephalic presentation the fetal head may be extended or flexed in a manner that presents a head diameter that exceeds the limits of the maternal pelvis, leading to prolonged labor, forceps- or vacuum-assisted birth, or cesarean birth.Ĭertain critical diameters of the fetal head are usually measured. 13-2).ĭeviations from the normal attitude may cause difficulties in childbirth. This attitude is termed general flexion (see Fig. ![]() The arms are crossed over the thorax, and the umbilical cord lies between the arms and the legs. Normally the back of the fetus is rounded so the chin is flexed on the chest, the thighs are flexed on the abdomen, and the legs are flexed at the knees. The fetus assumes a characteristic posture (attitude) in utero partly because of the mode of fetal growth and partly because of the way the fetus conforms to the shape of the uterine cavity. Factors that determine the presenting part include fetal lie, fetal attitude, and extension or flexion of the fetal head.Īttitude is the relation of the fetal body parts to one another. When the presenting part is the occiput, the presentation is noted as vertex (see Fig. In a cephalic presentation the presenting part is usually the occiput in a breech presentation it is the sacrum in the shoulder presentation it is the scapula. It is the part of the fetal body first felt by the examining finger during a vaginal examination. The presenting part is that part of the fetus that lies closest to the internal os of the cervix. 13-3, A-C) and shoulder presentation, seen in 1% of births (see Fig. 13-2) breech presentation (buttocks, feet, or both first), occurring in 3% of births ( Fig. The three main presentations are cephalic presentation (head first), occurring in 96% of births (Fig. Presentation refers to the part of the fetus that enters the pelvic inlet first and leads through the birth canal during labor at term. The circumference of the fetal hips is usually small enough to not create problems. This creates a shoulder diameter that is smaller than the skull, facilitating passage through the birth canal. Molding can be extensive, but the heads of most newborns assume their normal shape within 3 days after birth.Īlthough the size of the fetal shoulders may affect passage, their position can be altered relatively easily during labor so one shoulder may occupy a lower level than the other. This capacity of the bones to slide over one another also permits adaptation to the various diameters of the maternal pelvis. ![]() However, because the bones are not firmly united, slight overlapping, or molding of the shape of the head, occurs during labor. Sutures and fontanels make the skull flexible to accommodate the infant brain, which continues to grow for some time after birth. The posterior fontanel lies at the junction of the sutures of the two parietal bones and the occipital bone, is triangular, and is about 1 cm by 2 cm. The larger of these, the anterior fontanel, is diamond shaped, about 3 cm by 2 cm, and lies at the junction of the sagittal, coronal, and frontal sutures. The two most important fontanels are the anterior and posterior (see Fig.
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