Factors That Contribute to Normal
Labor— The Three Ps
Labor is the occurrence of uterine contractions of suffi-cient intensity, frequency, and duration to bring about demonstrable effacement and dilation of the cervix. Dystocia results from what have been categorized classi-cally as abnormalities of the “power” (uterine contractions or maternal expulsive forces), “passenger” (position, size, or presentation of the fetus), or “passage” (pelvis or soft tissues).
Uterine activity can be monitored by palpation, external tocody-namometry, or by using intrauterine pressure catheters (IUPCs) (Fig. 9.1). A tocodynamometer is an external strain gauge that is placed on the maternal abdomen. It records the frequency of uterine contractions and relaxations, as well as the duration of each contraction. An IUPC, in ad-dition to recording contraction frequency and duration, also directly measures the pressure generated by uterine contractions, via a catheter inserted into the uterine cav-ity. The catheter is attached to a gauge that measures intra-uterine pressure in millimeters of mercury (mm Hg).
Recent studies suggest that the use of an IUPC instead of external tocodynamometry does not affect the outcome in cases of abnormal labor.
However, an IUPC may be useful in specific situations, such as maternal obesity or other factors that may prevent accurate clinical evaluation of uterine contractions.
For cervical dilation and fetal descent to occur, each uterine contraction must generate at least 25 mm Hg of peak pressure. Optimal intrauterine pressure is 50 to 60 mm Hg. The frequency of uterine contractions is also impor-tant in generating a normal labor pattern: the optimal fre-quency of uterine contractions is a minimum of three contractions in a 10-minute interval, often described as “adequate.” Uterine contractions that are too frequent are not optimal, because they prevent intervals of uterine re-laxation. During this “rest interval,” the fetus receives unimpeded uteroplacental blood flow for oxygen and waste transport. Without these rest periods, fetal oxygenation may be compromised.
Another unit of measure commonly used to assess con-tractile strength is the Montevideo unit (MVU). This unit is the number of uterine contractions in 10 minutes times the average intensity (above the resting baseline intrauterine pressure). Normal progress of labor is usually associated with 200or more Montevideo units.
Evaluation of the passenger includes clinical estimation of fetal weight and clinical evaluation of fetal lie, presenta-tion, position, and attitude. If a fetus has an estimated weightgreater than 4000 to 4500 grams, the risk of dystocia, includ-ing shoulder dystocia and fetopelvic disproportion, is greater. Because ultrasound estimation of fetal weight is often in-accurate by as much as 500 to 1000 grams when the fetus is near term (40 weeks’ gestational age), this information must be used in conjunction with other parameters when making management decisions.
Fetal attitude, presentation, and lie also play a role in the progress of labor (Fig. 9.2). If the fetal head is asynclitic (turnedto one side; asynclitism) or extended (extension), a larger cephalic diameter is presented to the pelvis, thereby increas-ing the possibility of dystocia. A brow presentation (about 1 in 3000 deliveries) typically converts to either a vertex or face presentation, but, if persistent, may cause dystocia re-quiring cesarean delivery. Likewise, a face presentation (about 1 in 600 to 1000 deliveries) requires cesarean deliv-ery in most cases. However, a mentum anterior presenta-tion (chin toward mother’s abdomen) may be deliveredvaginally if the fetal head undergoes flexion, rather than the normal extension. A persistentoccipitoposterior positionis alsoassociated with longer labors (approximately 1 hour in multiparous patients and 2 hours in nulliparous patients). Incompoundpresentations, when one or more limbs prolapse alongsidethe presenting part (about 1 in 700 deliveries), the extrem-ity usually retracts (either spontaneously or with manual as-sistance) as labor continues. When it does not, or in the 15% to 20% of compound presentations associated with umbili-cal cord prolapse, cesarean delivery is required.
Fetal anomalies, such as hydrocephaly and soft tissue tumors, may also cause dystocia. The routine use of prena-tal ultrasound for other causes has allowed identification of these situations, significantly reducing the incidence of un-expected dystocia of this kind.
A number of maternal factors are associated with dystocia. Dystocia can result from maternal skeletal or soft-tissue anomalies that obstruct the birth canal. Cephalopelvicdisproportion, in which the size of the maternal pelvis isinadequate to the size of the presenting part of the fetus, may impede fetal descent into the birth canal.
Clinical, radiographic, and CT measurements of the bony pelvis are poor predictors of successful vaginal delivery, due to the inaccuracy of these measurements as well as case-by-case differences in fetal accommodation and mechanisms of labor.
Clinical pelvimetry, the manual evaluation of the diam-eters of the pelvis, is also a poor predictor of successful vaginal birth, except in rare circumstances when the pelvic diameters are so small as to render the pelvis “completely contracted.” Although radiographic and CT pelvimetry can be helpful in some cases, the progress of descent of the presenting part in labor is the best test of pelvic adequacy.
Soft-tissue causes of dystocia include abnormalities of the cervix, tumors or other lesions of the colon or adnexa, distended bladder, uterine fibroids, an accessory uterine horn, and morbid obesity. Epidural anesthesia may con-tribute to dystocia by decreasing the tone of the pelvic floor musculature.
Dystocia may be associated with serious complications for both the woman and the fetus. Infection (chorioamnionitis) is aconsequence of prolonged labor, especially in the setting of ruptured membranes. Fetal infection and bacteremia, including pneumonia caused by aspiration of infected amniotic fluid, is linked to prolonged labor. In addition, there are the attendant risks of cesarean or operative de-livery, such as maternal soft tissue injury to the lower genital tract and fetal trauma