The cause of the onset of labor in women—either term or preterm—remains unknown. In other mammalian species, a decrease in serum progesterone concentration in association with an increase in estrogen concentration is followed by increases in prostaglandin production, oxytocin receptors, and myometrial gap junction formation. In sheep, the fetus apparently triggers parturition through a surge in fetal cortisol production. In women, progesterone concentrations do not decline before the onset of labor and no surge in fetal cortisol secretion occurs. The laboring human uterus does manifest increases in prostaglandin production, oxytocin receptors, and myometrial gap junction formation.^1,2 As more is learned, perhaps a unifying concept of the onset of mammalian labor will emerge. Preterm and post-term deliveries both constitute important obstetric problems; and when more is understood about the mechanism of the onset of labor, new approaches to preventing the preterm and post-term onset of parturition may evolve.

Four pelvic types have been described on the basis of the shape of the pelvic inlet (the plane bounded by the upper inner pubic symphysis, the linea terminalis of the iliac bones, and the sacral promontory) (Table 18-1).³ The type and size of the pelvis constitute important predictors of the success of vaginal delivery.

Radiographic pelvimetry provides much more information regarding pelvic dimensions and features than can be obtained by clinical pelvimetry alone. However, it has only a limited place in clinical management because of its poor ability to predict a successful vaginal birth. In the absence of a history of pelvic fracture or musculoskeletal disease (e.g., a dwarfing condition), there are few circumstances in which the apparent pelvic anatomy precludes a trial of labor. A pelvis with smaller-than-average dimensions may be adequate for a particular fetus if the head is well-flexed, sufficient molding (i.e., overlapping of the unfused skull bones) has occurred, and the labor is strong; thus, radiographic pelvimetry does not always predict the presence or absence of cephalopelvic disproportion.⁴ Further, some risk is associated with radiographic pelvimetry. In addition to the potential for point mutations in the maternal oocytes and fetal germ cells, there is a small but apparently real increase in the incidence of malignancy and leukemia in children who were exposed to diagnostic radiation in utero. Some obstetricians use radiographic pelvimetry in cases of fetal breech presentation to assess whether fetal presentation, position, and lie are appropriate for vaginal delivery. The hope is to save a parturient from a long, futile labor and a hazardous delivery. Computed tomography and magnetic resonance imaging are associated with less or no ionizing radiation exposure, respectively; these methods are also more accurate than conventional radiographic pelvimetry. However, these methods also have limited ability to predict a successful vaginal delivery.

The position of the fetus denotes the relationship of a specific presenting fetal bony point to the maternal pelvis. In vertex presentations, that bony point is the occiput. During vaginal examination, palpation of the sagittal suture and fontanels permits determination of the fetal position. Positions of the occiput in early labor are listed in Box 18-1. Other markers for position are the sacrum for breech presentation, the mentum for face presentation, and the acromion for shoulder presentation. (See Chapter 35 for a discussion of nonvertex presentations.)

The mechanism of labor refers to the changes in fetal conformation and position (cardinal movements; Box 18-2) that occur during descent through the birth canal during the late first stage and the second stage of labor.

The first cardinal movement is engagement, which denotes passage of the biparietal diameter (BPD) (i.e., the widest transverse diameter of the fetal head) through the plane of the pelvic inlet. A direct clinical determination of engagement cannot be made, but obstetricians assume that engagement has occurred if the leading bony point of the fetal head is palpable at the level of the ischial spines. This is true because the distance between the leading bony point and the BPD is typically less than the distance between the ischial spines and the plane of the pelvic inlet. If the leading bony point is at the level of the spines, the vertex is said to be at zero station. If the leading bony point is 1 cm above the level of the spines, the station is designated as −1. Similarly, +1, +2, and +3 indicate that the leading bony point is 1, 2, and 3 cm below the ischial spines, respectively (Figure 18-1). At +5 station, delivery is imminent. Station refers to palpation of the leading bony point. Often marked edema of the scalp (i.e., caput succedaneum) occurs during labor. In such cases, the bony skull may be 2 to 3 cm higher than the scalp.

The second cardinal movement is descent, although it is artificial to separate descent from the other movements because descent occurs throughout the birth process. The third cardinal movement is flexion. A very small fetus can negotiate the average maternal pelvis without increased flexion. However, under the usual circumstances at term, the force from above and resistance from below enhance flexion of the occiput (Figure 18-2).

The next cardinal movement is extension, which occurs as the fetal head delivers (Figure 18-3). Subsequently, the occiput rotates to the side of the back (external rotation) as the shoulders pass through the midpelvis in an oblique diameter. The anterior shoulder moves under the pubic symphysis. With gentle downward traction, it passes from the birth canal, and expulsion of the remainder of the fetus occurs.

This description recounts events in the typical gynecoid pelvis. Abnormalities of the pelvis affect the mechanism of labor in specific ways. In an anthropoid pelvis, the anteroposterior diameter of the pelvic inlet exceeds the transverse diameter. Often internal rotation to the occiput posterior position rather than the occiput anterior position occurs. Because the pelvis is narrow transversely, further descent of the vertex occurs with the occiput in the posterior position. Delivery occurs with the occiput in the posterior position, or rotation to the occiput anterior position occurs just before delivery. In cases of persistent occiput posterior position, delivery occurs by flexion rather than extension of the fetal head (see Figure 18-3).

In platypelloid pelves, internal rotation may not take place. The widest diameter is the transverse diameter, and descent of the vertex may occur with the occiput in the transverse position; rotation to the occiput anterior position occurs only at delivery.

Admission

When a patient enters the labor and delivery unit, the first question that must be asked is “why?” Did she come because of regular, painful uterine contractions; decreased fetal activity; vaginal bleeding; ruptured membranes; or some other reason? If the tentative diagnosis is labor, is she at term?

The time of the onset of labor and the presumed status of the membranes should be determined. Observation of the patient’s demeanor coupled with the assessment of cervical effacement and dilation will signal whether the patient is in early or advanced labor. Examination of the cervix is deferred in patients with vaginal bleeding in the second half of pregnancy, unless placenta previa has been ruled out by ultrasonography, to avoid exacerbation of bleeding. To prevent infection, cervical examination may also be deferred in patients with premature rupture of membranes and no labor.

The obstetrician also directs attention to the second patient: the fetus. Abdominal examination or ultrasonography is used to establish presentation and an estimate of fetal size. With most obstetric services, external electronic fetal heart rate (FHR) monitoring is used on admission to assess fetal condition. The baseline rate and variability and the presence or absence of accelerations and decelerations are of interest.

The maternal vital signs and FHR are recorded periodically. In some obstetric services, continuous electronic FHR monitoring is used universally; with other services, it is monitored via intermittent auscultation. In low-risk patients, recording the FHR every 30 minutes in the first part of the first stage of labor, every 15 minutes in the latter part of the first stage, and every 5 minutes in the second stage is perfectly acceptable. During early labor, the patient may ambulate or assume any position of comfort on the labor bed or in a chair. During advanced labor, many women choose to lie down. Choices concerning analgesia or anesthesia are made according to the patient’s wishes. Figure 18-4 shows a flow sheet (partogram) that may be useful for charting the course of labor.

A generation of obstetricians is indebted to Emanuel Friedman, whose landmark studies of labor provide a framework for judging labor progress. Friedman’s approach was straightforward: He graphed cervical dilation on the y-axis and elapsed time on the x-axis for thousands of labors. He considered nulliparous and parous patients separately, and he determined the statistical limits of normal.⁵ The curve of cervical dilation over time is sigmoid shaped (Figure 18-5).

Most authorities consider Friedman’s most important contribution to be his separation of the latent phase from the active phase of the first stage of labor. Many hours of regular, painful uterine contractions may take place with little appreciable change in the cervix. During this latent (or preparatory) phase, the cervix may efface and become softer. Quite abruptly, the active (or dilation) phase begins, and regular increases in cervical dilation are expected over time. The transition from the latent to the active phase of the first stage of labor does not occur at an arbitrary cervical dilation but rather is known—in retrospect—by change in slope of the cervical dilation curve. Peisner and Rosen⁶ evaluated the progress of labor for 1060 nulliparous women and 639 parous women. After excluding women with protracted or arrested labor, these researchers noted that 60% of the women had reached the latent-active phase transition by 4 cm of cervical dilation and 89% did so by 5 cm.

A nulliparous woman may labor for 20 hours without achieving appreciable cervical dilation; 14 hours is the limit of the latent phase in the parous woman. Difficulty in assigning length to the latent phase lies not with its end (determined from the change in slope of the cervical dilation curve) but rather with its beginning. The onset of labor is self-reported by the parturient. The uterus contracts throughout gestation, and the level of prelabor uterine activity and its perception are variable. Often both the patient and the physician are uncertain as to exactly when labor started.

According to Friedman, in the active phase of the first stage of labor, a nulliparous woman’s cervix should dilate at a rate of at least 1.2 cm per hour and a parous woman’s cervix should dilate at least 1.5 cm per hour. (The slopes of the dilation curves in Figure 18-5 represent the lower limits of normal.) If a woman’s cervix fails to dilate at the appropriate rate during the active phase of labor, she is said to have primary dysfunctional labor. Graphically, her cervical dilation “falls off the curve.” If cervical dilation ceases during a 2-hour period in the active phase of labor, secondary arrest of dilation has occurred.

More recent studies have reported slower rates of cervical dilation and engendered an ongoing transition toward the use of the “contemporary labor curve.”^7,8 These curves reveal that cervical dilation is particularly slow prior to 6 cm and that the deceleration phase described by Friedman is usually absent. Therefore, 6 cm rather than 4 cm of cervical dilation more accurately reflects the start of the active phase in contemporary labor curves.⁸ Furthermore, in the active phase, absence of cervical change over at least 4 hours rather than 2 hours is a better definition of labor arrest. Estimates of contemporary rates of cervical dilation from 4 cm (when patients are often admitted) by parity are presented (Table 18-2). Nulliparous women have a slower cumulative rate of cervical dilation overall. However, prior to 6 cm of dilation, the times required to dilate 1 cm are similar between nulliparous and parous women.

Abnormalities of the latent phase and active phase differ in associated factors, apparent causes, and significance. A prolonged latent phase is more likely if labor begins “before the cervix is ready.”⁵ Just as there is a wide range of prelabor uterine activity, so too is there a wide range of cervical softness, effacement, and dilation at the start of labor. In some women, appreciable cervical softening, effacement, and dilation take place in late pregnancy; thus, when clinical labor begins, the cervix may already be 3 to 4 cm dilated and completely effaced. Alternatively, in other women, there is no cervical effacement or dilation at the start of labor. Given these differences, it is not surprising that varying amounts of uterine contractile work are needed to cause dilation of the cervix. The most common factor associated with a prolonged latent phase is an “unripe” cervix at the start of labor. Some women with a prolonged latent phase are not in true labor at all but are in “false labor”; this diagnosis is made in retrospect. After hours of regular, painful contractions, uterine activity may cease without the occurrence of appreciable cervical dilation. Several hours or days later, the patient reappears in true labor. During the latent phase of labor, it is not known with certainty whether a woman is in true or false labor.

Recent studies refute the dictum that a prolonged latent phase alone is not associated with fetal compromise or cephalopelvic disproportion.^9,10 An increased risk for cesarean delivery, chorioamnionitis, endometritis, excessive blood loss, depressed Apgar scores, and need for neonatal resuscitation have all been associated with a prolonged latent phase. An ongoing increase in the use of labor induction may be a mediating factor. Primary dysfunctional labor and arrest of dilation during the active phase may also indicate cephalopelvic disproportion.⁵

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