Normal Labor and Delivery

Uterine Activity (Powers)

The powers refer to the forces generated by the uterine musculature. Uterine activity is characterized by the frequency, amplitude (intensity), and duration of contractions. Assessment of uterine activity may include simple observation, manual palpation, external objective assessment techniques (such as external tocodynamometry), and direct measurement via an intrauterine pressure catheter (IUPC). External tocodynamometry measures the change in shape of the abdominal wall as a function of uterine contractions and, as such, is qualitative rather than quantitative. Although it permits graphic display of uterine activity and allows for accurate correlation of fetal heart rate (FHR) patterns with uterine activity, external tocodynamometry does not allow measurement of contraction intensity or basal intrauterine tone. The most precise method for determination of uterine activity is the direct measurement of intrauterine pressure with an IUPC. However, this procedure should not be performed unless indicated given the small but finite associated risk of maternal fever. Several historical case reports document additional complications, including placental disruption, uterine perforation, and fetal-maternal hemorrhage, which were associated with first-generation versions of this device.

Despite technologic improvements, the definition of “adequate” uterine activity during labor remains unclear. Classically, three to five contractions in 10 minutes has been used to define adequate labor; this pattern has been observed in approximately 95% of women in spontaneous labor. In labor, patients usually contract every 2 to 5 minutes, with contractions becoming as frequent as every 2 to 3 minutes in late active labor and during the second stage. Abnormal uterine activity can also be observed either spontaneously or as a result of iatrogenic interventions. Tachysystole is defined as more than five contractions in 10 minutes averaged over 30 minutes. If tachysystole occurs, documentation should note the presence or absence of FHR decelerations. The term hyperstimulation should no longer be used.

Various units of measure have been devised to objectively quantify uterine activity, the most common of which is the Montevideo unit (MVU) , a measure of average frequency and amplitude above basal tone (the average strength of contractions in millimeters of mercury multiplied by the number of contractions per 10 minutes). Although 150 to 350 MVU has been described for adequate labor, 200 to 250 MVU is commonly accepted to define adequate labor in the active phase. No data identify adequate forces during latent labor. Although it is generally believed that optimal uterine contractions are associated with an increased likelihood of vaginal delivery, data are limited to support this assumption. If uterine contractions are “adequate” to effect vaginal delivery, one of two things will happen: either the cervix will efface and dilate and the fetal head will descend, or caput succedaneum (scalp edema) and molding of the fetal head (overlapping of the skull bones) will worsen without cervical effacement and dilation. The latter situation suggests the presence of cephalopelvic disproportion (CPD), which can be either absolute , in which the fetus is simply too large to negotiate the pelvis, or relative , in which delivery of the fetus through the pelvis would be possible under optimal conditions but is precluded by malposition or abnormal attitude of the fetal head.

Fetus (Passenger)

The passenger, of course, is the fetus. Several fetal variables influence the course of labor and delivery. Fetal size can be estimated clinically by abdominal palpation or ultrasound or by asking a multiparous patient about her best estimate, but all of these methods are subject to a large degree of error. Fetal macrosomia is defined by the American College of Obstetricians and Gynecologists (ACOG) as birthweight greater than 4000 g to 4500 g for any gestational age, and it is associated with an increased likelihood of planned cesarean delivery, labor dystocia, cesarean delivery (CD) after a failed trial of labor, shoulder dystocia, and birth trauma. Fetal lie refers to the longitudinal axis of the fetus relative to the longitudinal axis of the uterus. Fetal lie can be longitudinal, transverse, or oblique ( Fig. 11.3 ). In a singleton pregnancy, only fetuses in a longitudinal lie can be safely delivered vaginally.

Presentation refers to the fetal part that directly overlies the pelvic inlet. In a fetus presenting in the longitudinal lie, the presentation can be cephalic (vertex) or breech. Compound presentation refers to the presence of more than one fetal part overlying the pelvic inlet, such as a fetal hand and the vertex. Funic presentation refers to presentation of the umbilical cord and is rare at term. In a cephalic fetus, the presentation is classified according to the leading bony landmark of the skull, which can be either the occiput (vertex), the chin (mentum), or the brow ( Fig. 11.4 ). Malpresentation , a term that refers to any presentation other than vertex, is seen in approximately 5% of all term labors (see Chapter 17 ).

Attitude refers to the position of the head with regard to the fetal spine (the degree of flexion and/or extension of the fetal head). Flexion of the head is important to facilitate engagement of the head in the maternal pelvis. When the fetal chin is optimally flexed onto the chest, the suboccipitobregmatic diameter (9.5 cm) presents at the pelvic inlet ( Fig. 11.5 ). This is the smallest possible presenting diameter in the cephalic presentation. As the head deflexes (extends), the diameter presenting to the pelvic inlet progressively increases even before the malpresentations of brow and face are encountered (see Fig. 11.5 ) and may contribute to failure to progress in labor. The architecture of the pelvic floor along with increased uterine activity may correct deflexion in the early stages of labor.

Position of the fetus refers to the relationship of the fetal presenting part to the maternal pelvis, and it can be assessed most accurately on vaginal examination. For cephalic presentations, the fetal occiput is the reference: if the occiput is directly anterior, the position is occiput anterior (OA); if the occiput is turned toward the mother's right side, the position is right occiput anterior (ROA). In the breech presentation, the sacrum is the reference (right sacrum anterior). The various positions of a cephalic presentation are illustrated in Fig. 11.6 . In a vertex presentation, position can be determined by palpation of the fetal sutures: the sagittal suture is the easiest to palpate, but palpation of the distinctive lambdoid sutures should identify the position of the fetal occiput; the frontal suture can also be used to determine the position of the front of the vertex.

Most commonly, the fetal head enters the pelvis in a transverse position and then, as a normal part of labor, it rotates to an OA position. Most fetuses deliver in the OA, left occiput anterior (LOA), or ROA position. Malposition refers to any position in labor that is not in the above three categories. In the past, fewer than 10% of presentations were occiput posterior (OP) at delivery. However, epidural analgesia may be an independent risk factor for persistent OP presentation in labor. In an observational cohort study, OP presentation was observed in 12.9% of women with epidurals compared with 3.3% of controls ( P = .002). In a Cochrane meta-analysis of four randomized controlled trials (RCTs), malposition was 40% more likely for women with an epidural compared with controls; however, this difference was not statistically significant, and more RCTs are needed (odds ratio [OR], 1.40; 95% confidence interval [CI], 0.98 to 1.99). Asynclitism occurs when the sagittal suture is not directly central relative to the maternal pelvis. If the fetal head is turned such that more parietal bone is present posteriorly, the sagittal suture is more anterior; this is referred to as posterior asynclitism. In contrast, anterior asynclitism occurs as more parietal bone presents anteriorly. The occiput transverse (OT) and OP positions are less common at delivery and are more difficult to deliver.

Station is a measure of descent of the bony presenting part of the fetus through the birth canal ( Fig. 11.7 ). The current standard classification (−5 to +5) is based on a quantitative measure in centimeters of the distance of the leading bony edge from the ischial spines. The midpoint (0 station) is defined as the plane of the maternal ischial spines. The ischial spines can be palpated on vaginal examination at approximately 8 o'clock and 4 o'clock. For the right-handed person, they are most easily felt on the maternal right.

An abnormality in any of these fetal variables may affect both the course of labor and the route of delivery. For example, OP presentation is well known to be associated with longer labor, operative vaginal delivery (OVD), and an increased risk of CD.

Maternal Pelvis (Passage)

The passage consists of the bony pelvis—composed of the sacrum, ilium, ischium, and pubis—and the resistance provided by the soft tissues. The bony pelvis is divided into the false (greater) and true (lesser) pelvis by the pelvic brim, which is demarcated by the sacral promontory, the anterior ala of the sacrum, the arcuate line of the ilium, the pectineal line of the pubis, and the pubic crest culminating in the symphysis ( Fig. 11.8 ). Measurements of the various parameters of the bony female pelvis have been made with great precision, directly in cadavers and using radiographic imaging in living women. Such measurements have divided the true pelvis into a series of planes that must be negotiated by the fetus during passage through the birth canal, which can be broadly termed the pelvic inlet, midpelvis, and pelvic outlet. Pelvimetry performed with radiographic computed tomography (CT) or magnetic resonance imaging (MRI) has been used to determine average and critical limit values for the various parameters of the bony pelvis. Critical limit values are measurements that may be associated with a significant probability of CPD depending upon fetal size and gestational age. However, subsequent studies were unable to demonstrate threshold pelvic or fetal cutoff values with sufficient sensitivity or specificity to predict CPD and the subsequent need for CD prior to the onset of labor. In current obstetric practice, radiographic CT and MRI pelvimetry are rarely used given the lack of evidence of benefit and some data that show possible harm (increased incidence of cesarean delivery); instead, a clinical trial of the pelvis (labor) is used. The remaining indications for radiography, CT pelvimetry, or MRI are evaluation for vaginal breech delivery or evaluation of a woman who has suffered a significant pelvic fracture.

Clinical pelvimetry is currently the only method of assessing the shape and dimensions of the bony pelvis in labor. A useful protocol for clinical pelvimetry is detailed in Fig. 11.9 and involves assessment of the pelvic inlet, midpelvis, and pelvic outlet. Reported average and critical-limit pelvic diameters may be used as a historical reference during the clinical examination to determine pelvic shape and assess risk for CPD. The inlet of the true pelvis is largest in its transverse diameter and averages 13.5 cm. The diagonal conjugate, the distance from the sacral promontory to the inferior margin of the symphysis pubis as assessed on vaginal examination, is a clinical representation of the anteroposterior (AP) diameter of the pelvic inlet. The true conjugate, or obstetric conjugate, of the pelvic inlet is the distance from the sacral promontory to the superior aspect of the symphysis pubis. The obstetric conjugate has an average value of 11 cm and is the smallest diameter of the inlet. It is considered to be contracted if it measures less than 10 cm. The obstetric conjugate cannot be measured clinically but can be estimated by subtracting 1.5 to 2.0 cm from the diagonal conjugate, which has an average distance of 12.5 cm.

The limiting factor in the midpelvis is the transverse interspinous diameter (the measurement between the ischial spines), which is usually the smallest diameter of the pelvis but should be greater than 10 cm. The pelvic outlet is rarely of clinical significance, however. The average pubic angle is greater than 90 degrees and will typically accommodate two fingerbreadths. The AP diameter from the coccyx to the symphysis pubis is approximately 13 cm in most cases, and the transverse diameter between the ischial tuberosities is approximately 8 cm and will typically accommodate four knuckles (see Fig. 11.9 ).

The shape of the female bony pelvis can be classified into four broad categories: gynecoid, anthropoid, android, and platypelloid ( Fig. 11.10 ). This classification is based on the radiographic studies of Caldwell and Moloy and separates those with more favorable characteristics (gynecoid, anthropoid) from those less favorable for vaginal delivery (android, platypelloid). In reality, however, many women fall into intermediate classes, and the distinctions become arbitrary. The gynecoid pelvis is the classic female shape. The anthropoid pelvis—with its exaggerated oval shape of the inlet, largest AP diameter, and limited anterior capacity—is more often associated with delivery in the OP position. The android pelvis is male in pattern and theoretically has an increased risk of CPD, and the broad and flat platypelloid pelvis theoretically predisposes to a transverse arrest. Although the assessment of fetal size, along with pelvic shape and capacity, is still of clinical utility, it is a very inexact science . An adequate trial of labor is the only definitive method to determine whether a fetus will be able to safely negotiate through the pelvis.

Pelvic soft tissues may provide resistance in both the first and second stages of labor. In the first stage, resistance is offered primarily by the cervix, whereas in the second stage, it is offered by the muscles of the pelvic floor. In the second stage of labor, the resistance of the pelvic musculature is believed to play an important role in the rotation and movement of the presenting part through the pelvis.

Engagement

Engagement refers to passage of the widest diameter of the presenting part to a level below the plane of the pelvic inlet ( Fig. 11.12 ). In the cephalic presentation with a well-flexed head, the largest transverse diameter of the fetal head is the biparietal diameter (9.5 cm). In the breech, the widest diameter is the bitrochanteric diameter. Clinically, engagement can be confirmed by palpation of the presenting part both abdominally and vaginally. With a cephalic presentation, engagement is achieved when the leading bony presenting part is at zero station on vaginal examination. Engagement is considered an important clinical prognostic sign because it demonstrates that, at least at the level of the pelvic inlet, the maternal bony pelvis is sufficiently large to allow descent of the fetal head. In nulliparas, engagement of the fetal head usually occurs by 36 weeks’ gestation; however, in multiparas engagement can occur later in gestation or even during the course of labor.