he myometrium of the upper segment does not relax to its original length after contractions. Instead, it becomes relatively fixed at a shorter length. The upper active uterine segment con tracts down on its diminishing contents, but myometrial ten sion remains constant. he net efect is to take up slack, thus maintaining the advantage gained in expulsion of the fetus.
Concurrently, the uterine musculature is kept in irm contact with the uterine contents. As the consequence of retraction, left of. hus, the upper part of the uterine cavity becomes slightly smaller with each successive contraction. Because of the successive shortening of the muscular fibers, the upper active second-stage labor (see Fig. 21-11). This process continues and immediately after delivery.
Clinically, it is important to understand that the phenomenon of upper segment retraction is contingent on a decrease in the volume of its contents. For this to happen, particularly early in labor when the entire uterus is virtually a closed sac with only minimal cervical dilation, the musculature of the lower segment must stretch. This permits a greater portion of the uterine contents to occupy the lower segment. The upper segment retracts only to the extent that the lower segment distends and the cervix dilates.
Relaxation of the lower uterine segment mirrors the same gradual progression of retraction. Recall that after each contraction of the upper segment, the muscles do not return to their previous length, but tension remains essentially the same. By comparison, in the lower segment, successive lengthening of the fibers with labor is accompanied by thinning, normally to only a few millimeters in the thinnest part. s a result of the lower segment thinning and concomitant upper segment thickening, a boundary between the two is marked by a ridge on the inner uterine surface-the physiological retraction ring. When the thinning of the lower uterine segment is extreme, as in obstructed labor, the ring is prominent and forms a pathological retraction ring. This abnormal condition is also known as the Bandl ring, which is discussed further in Chapter 23 (p. 455).
Changes in Uterine Shape. Each contraction gradually elongates the ovoid uterine shape and thereby narrows the horizontal diameter. This change in shape has important efects on the labor process. First, there is greater etal xis pressure, that is, the smaller horizontal diameter serves to straighten the fetal vertebral column. This presses the upper pole of the fetus firmly against the fundus, whereas the lower pole is thrust farther downward. The lengthening of the ovoid shape has been estimated at 5 to 10 cm. Second, with lengthening of the uterus, the longitudinal muscle ibers are drawn taut. As a result, the lower segment and cervix are the only parts of the uterus that are flexible, and these are pulled upward and around the lower pole of the fetus.
After the cervix is dilated fully, the most important force in fetal expulsion is produced by maternal intraabdominal pressure.
Physiology of Labor 413
Contraction of the abdominal muscles simultaneously with forced respiratory eforts with the glottis closed is referred to as pushing. The force is similar to that with defecation, but the intensity usually is much greater. The importance of intraabdominal pressure is shown by the prolonged descent during labor in paraplegic women and in those with a dense epidural block. And, although increased intraabdominal pressure is necessary to complete second-stage labor, pushing accomplishes little in the first stage. It exhausts the mother, and its associated elevated intrauterine pressures may be harmful to the fetus.
s the result of contraction forces, two fundamental changesefacement and dilation-occur in the ripened cervix. For an average-sized fetal head to pass through the cervix, its canal must dilate to a diameter of approximately 10 cm. At this time, the cervix is said to be completely or ully dilated. Although there may be no fetal descent during cervical efacement, most commonly the presenting fetal part descends somewhat as the cervix dilates.