Key Abbreviations
American Academy of Pediatrics AAP
American College of Obstetricians and Gynecologists ACOG
Body mass index BMI
California Maternal Quality Care Collaborative CMQCC
Cephalopelvic disproportion CPD
Cervical length CL
Computed tomography CT
Dehydroepiandrosterone sulfate DHEAS
Fetal heart rate FHR
Intrauterine pressure catheter IUPC
Intraventricular hemorrhage IVH
Left occiput anterior LOA
Magnetic resonance imaging MRI
Montevideo unit MVU
Normal saline NS
Occiput anterior OA
Occiput posterior OP
Occiput transverse OT
Prostaglandin PG
Randomized controlled trial RCT
Right occiput anterior ROA
Skin-to-skin contact SSC
Society for Maternal-Fetal Medicine SMFM
Overview
The initiation of normal labor at term requires endocrine, paracrine, and autocrine signaling between the fetus, uterus, placenta, and the mother. Although the exact trigger for human labor at term remains unknown, it is believed to involve conversion of fetal dehydroepiandrosterone sulfate (DHEAS) to estriol and estradiol by the placenta. These hormones upregulate transcription of progesterone, progesterone receptors, oxytocin receptors, and gap junction proteins within the uterus, which helps to facilitate regular uterine contractions. The latent phase of labor is characterized by a slower rate of cervical dilation, whereas the active phase of labor is characterized by a faster rate of cervical dilation and does not begin for most women until the cervix is dilated 6 cm. The duration of the second stage of labor can be affected by a number of factors including epidural use, fetal position, fetal weight, ethnicity, and parity. This chapter will review the characteristics and physiology of normal labor at term. Factors that affect the average duration of the first and second stage of labor progress will be reviewed, and an evidence-based evaluation of strategies to support the mother during labor and facilitate safe delivery of the fetus will be presented.
Labor: Definition and Physiology
Labor is defined as the process by which the fetus is expelled from the uterus. More specifically, labor requires regular, effective contractions that lead to dilation and effacement of the cervix. This chapter describes the physiology and normal characteristics of term labor and delivery.
The physiology of labor initiation has not been completely elucidated, but the putative mechanisms have been well reviewed by Liao and colleagues. Labor initiation is species specific, and the mechanisms of human labor are unique. The four phases of labor from quiescence to involution are outlined in Figure 12-1 . The first phase is quiescence, which represents that time in utero before labor begins, when uterine activity is suppressed by the action of progesterone, prostacyclin, relaxin, nitric oxide, parathyroid hormone–related peptide, and possibly other hormones. During the activation phase , estrogen begins to facilitate expression of myometrial receptors for prostaglandins (PGs) and oxytocin, which results in ion channel activation and increased gap junctions. This increase in the gap junctions between myometrial cells facilitates effective contractions. In essence, the activation phase readies the uterus for the subsequent stimulation phase , when uterotonics—particularly PGs and oxytocin—stimulate regular contractions. In the human, this process at term may be protracted, occurring over days to weeks. The final phase, uterine involution , occurs after delivery and is mediated primarily by oxytocin. The first three phases of labor require endocrine, paracrine, and autocrine interaction between the fetus, membranes, placenta, and mother.
The fetus has a central role in the initiation of term labor in nonhuman mammals; in humans, the fetal role is not completely understood ( Fig. 12-2 ). In sheep, term labor is initiated through activation of the fetal hypothalamic-pituitary-adrenal (HPA) axis, with a resultant increase in fetal adrenocorticotropic hormone (ACTH) and cortisol. Fetal cortisol increases production of estradiol and decreases production of progesterone by a shift in placental metabolism of cortisol dependent on placental 17α-hydroxylase. The change in the circulating progesterone/estradiol concentration stimulates placental production of oxytocin and PG, particularly PGF 2α , which in turn promotes myometrial contractility. If this increase in fetal ACTH and cortisol is blocked, progesterone levels remain unchanged, and parturition is delayed. In contrast, humans lack placental 17α-hydroxylase, maternal and fetal levels of progesterone remain elevated, and no trigger exists for parturition because of an increase in fetal cortisol near term. Rather, in humans, evidence suggests that placental production of corticotropin-releasing hormone (CRH) near term activates the fetal hypothalamic-pituitary axis and results in increased production of dehydroepiandrostenedione by the fetal adrenal gland. Fetal dehydroepiandrostenedione is converted in the placenta to estradiol and estriol. Placenta-derived estriol potentiates uterine activity by enhancing the transcription of maternal (likely decidual) PGF 2α , PG receptors, oxytocin receptors, and gap-junction proteins. In humans, no documented decrease in progesterone has been observed near term, and a fall in progesterone is not necessary for labor initiation. However, some research suggests the possibility of a functional progesterone withdrawal in humans. Labor is accompanied by a decrease in the concentration of progesterone receptors and a change in the ratio of progesterone receptor isoforms A and B in both the myometrium and the membranes. During labor, increased expression of nuclear and membrane progesterone receptor isoforms serve to enhance genomic expression of contraction-associated proteins, increase intracellular calcium, and decrease cyclic adenosine monophosphate (cAMP). More research is needed to elucidate the precise mechanism through which the human parturition cascade is activated. Fetal maturation might play an important role as might maternal cues that affect circadian cycling. Most species have distinct diurnal patterns of contractions and delivery, and in humans, the majority of contractions occur at night.
Oxytocin is commonly used for labor induction and augmentation, and a full understanding of the mechanism of oxytocin action is important. Oxytocin is a peptide hormone synthesized in the hypothalamus and released from the posterior pituitary in a pulsatile fashion. At term, oxytocin serves as a potent uterotonic agent capable of stimulating uterine contractions at intravenous (IV) infusion rates of 1 to 2 mIU/min. Oxytocin is inactivated largely in the liver and kidney, and during pregnancy, it is degraded primarily by placental oxytocinase. Its biologic half-life is approximately 3 to 4 minutes, but it appears to be shorter when higher doses are infused. Concentrations of oxytocin in the maternal circulation do not change significantly during pregnancy or before the onset of labor, but they do rise late in the second stage of labor. Studies of fetal pituitary oxytocin production and the umbilical arteriovenous differences in plasma oxytocin strongly suggest that the fetus secretes oxytocin that reaches the maternal side of the placenta. The calculated rate of active oxytocin secretion from the fetus increases from a baseline of 1 mIU/min before labor to around 3 mIU/min after spontaneous labor.
Significant differences in myometrial oxytocin receptor distribution have been reported, with large numbers of fundal receptors and fewer receptors in the lower uterine segment and cervix. Myometrial oxytocin receptors increase on average by 100- to 200-fold during pregnancy and reach a maximum during early labor. This rise in receptor concentration is paralleled by an increase in uterine sensitivity to circulating oxytocin. Specific high-affinity oxytocin receptors have also been isolated from human amnion and decidua parietalis but not decidua vera. It has been suggested that oxytocin plays a dual role in parturition. First, through its receptor, oxytocin directly stimulates uterine contractions. Second, oxytocin may act indirectly by stimulating the amnion and decidua to produce PG. Indeed, even when uterine contractions are adequate, induction of labor at term is successful only when oxytocin infusion is associated with an increase in PGF production.
Oxytocin binding to its receptor activates phospholipase C. In turn, phospholipase C increases intracellular calcium both by stimulating the release of intracellular calcium and by promoting the influx of extracellular calcium. Oxytocin stimulation of phospholipase C can be inhibited by increased levels of cAMP. Increased calcium levels stimulate the calmodulin-mediated activation of myosin light-chain kinase. Oxytocin may also stimulate uterine contractions via a calcium-independent pathway by inhibiting myosin phosphatase, which in turn increases myosin phosphorylation. These pathways (of PGF 2α and intracellular calcium) have been the target of multiple tocolytic agents: indomethacin, calcium channel blockers, β-mimetics (through stimulation of cAMP), and magnesium.
Mechanics of Labor
Labor and delivery are not passive processes in which uterine contractions push a rigid object through a fixed aperture. The ability of the fetus to successfully negotiate the pelvis during labor and delivery depends on the complex interactions of three variables: uterine activity, the fetus, and the maternal pelvis. This complex relationship has been simplified in the mnemonic powers, passenger, passage .
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 risks of uterine perforation, placental disruption, and intrauterine infection.
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 tachysytole 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 min). 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 or equal to the 90th percentile for a given gestational age or greater than 4500 g for any gestational age, and it is associated with an increased likelihood of planned cesarean delivery, labor dystocia, cesarean delivery 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. 12-3 ). 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. 12-4 ). 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. 12-5 ). 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. 12-5 ) 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 Figure 12-6 . 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 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. 12-7 ). 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, and an increased risk of cesarean delivery.
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. 12-8 ). 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 ( Table 12-1 ). 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 cesarean delivery 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.
| DIAMETER | AVERAGE VALUE | CRITICAL LIMIT * |
|---|---|---|
| Pelvic Inlet | ||
| Anteroposterior (cm) | 12.5 | 10.0 |
| Transverse (cm) | 13.0 | 12.0 |
| Sum (cm) | 25.5 | 22.0 |
| Area (cm 2 ) | 145.0 | 123.0 |
| Pelvic Midcavity | ||
| Anteroposterior (cm) | 11.5 | 10.0 |
| Transverse (cm) | 10.5 | 9.5 |
| Sum (cm) | 22.0 | 19.5 |
| Area (cm 2 ) | 125.0 | 106.0 |
* The critical limit values cited imply a high likelihood of cephalopelvic disproportion.
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 Figure 12-9 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. 12-9 ).
The shape of the female bony pelvis can be classified into four broad categories: gynecoid, anthropoid, android, and platypelloid ( Fig. 12-10 ). 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.
Cardinal Movements in Labor
The cardinal movements refer to changes in the position of the fetal head during its passage through the birth canal. Because of the asymmetry of the shape of both the fetal head and the maternal bony pelvis, such rotations are required for the fetus to successfully negotiate the birth canal. Although labor and birth comprise a continuous process, seven discrete cardinal movements are described: (1) engagement, (2) descent, (3) flexion, (4) internal rotation, (5) extension, (6) external rotation or restitution, and (7) expulsion ( Fig. 12-11 ).
Engagement
Engagement refers to passage of the widest diameter of the presenting part to a level below the plane of the pelvic inlet ( Fig. 12-12 ). 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 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.
Descent
Descent refers to the downward passage of the presenting part through the pelvis. Descent of the fetus is not continuous; the greatest rates of descent occur in the late active phase and during the second stage of labor.
Flexion
Flexion of the fetal head occurs passively as the head descends owing to the shape of the bony pelvis and the resistance offered by the soft tissues of the pelvic floor. Although flexion of the fetal head onto the chest is present to some degree in most fetuses before labor, complete flexion usually occurs only during the course of labor. The result of complete flexion is to present the smallest diameter of the fetal head (the suboccipitobregmatic diameter) for optimal passage through the pelvis.
Internal Rotation
Internal rotation refers to rotation of the presenting part from its original position as it enters the pelvic inlet (usually OT) to the AP position as it passes through the pelvis. As with flexion, internal rotation is a passive movement that results from the shape of the pelvis and the pelvic floor musculature. The pelvic floor musculature, including the coccygeus and ileococcygeus muscles, forms a V -shaped “hammock” that diverges anteriorly. As the head descends, the occiput of the fetus rotates toward the symphysis pubis—or, less commonly, toward the hollow of the sacrum—thereby allowing the widest portion of the fetus to negotiate the pelvis at its widest dimension. Owing to the angle of inclination between the maternal lumbar spine and pelvic inlet, the fetal head engages in an asynclitic fashion (i.e., with one parietal eminence lower than the other). With uterine contractions, the leading parietal eminence descends and is first to engage the pelvic floor. As the uterus relaxes, the pelvic floor musculature causes the fetal head to rotate until it is no longer asynclitic.
Extension
Extension occurs once the fetus has descended to the level of the introitus. This descent brings the base of the occiput into contact with the inferior margin at the symphysis pubis. At this point, the birth canal curves upward. The fetal head is delivered by extension and rotates around the symphysis pubis. The forces responsible for this motion are the downward force exerted on the fetus by the uterine contractions along with the upward forces exerted by the muscles of the pelvic floor.
External Rotation
External rotation, also known as restitution, refers to the return of the fetal head to the correct anatomic position in relation to the fetal torso. This can occur to either side depending on the orientation of the fetus; this is again a passive movement that results from a release of the forces exerted on the fetal head by the maternal bony pelvis and its musculature and mediated by the basal tone of the fetal musculature.
Expulsion
Expulsion refers to delivery of the rest of the fetus. After delivery of the head and external rotation, further descent brings the anterior shoulder to the level of the symphysis pubis. The anterior shoulder is delivered in much the same manner as the head, with rotation of the shoulder under the symphysis pubis . After the shoulder, the rest of the body is usually delivered without difficulty.
Normal Progress of Labor
Progress of labor is measured with multiple variables. With the onset of regular contractions, the fetus descends in the pelvis as the cervix both effaces and dilates. With each vaginal examination to judge labor progress, the clinician must assess not only cervical effacement and dilation but fetal station and position. This assessment depends on skilled digital palpation of the maternal cervix and the presenting part. As the cervix dilates in labor, it thins and shortens—or becomes more effaced —over time. Cervical effacement refers to the length of the remaining cervix and can be reported in length or as a percentage. If percentage is used, 0% effacement at term refers to at least a 2 cm long or a very thick cervix, and 100% effacement refers to no length remaining or a very thin cervix. Most clinicians use percentages to follow cervical effacement during labor. Generally, 80% or greater effacement is observed in women who are in active labor. Dilation, perhaps the easiest assessment to master, ranges from closed (no dilation) to complete (10 cm dilated). For most women, a cervical dilation that accommodates a single index finger is equal to 1 cm, and two index fingers’ dilation is equal to 3 cm. If no cervix can be palpated around the presenting part, the cervix is 10 cm or completely dilated. The assessment of station, discussed earlier, is important for documentation of progress, but it is also critical when determining if an operative vaginal delivery is feasible. Fetal head position should be regularly determined once the woman is in active labor; ideally, this should occur before significant caput has developed, which obscures the sutures. Like station, knowledge of the fetal position is critical before performing an operative vaginal delivery (see Chapter 14 ).
Labor occurs in three stages: the first stage is from labor onset until full dilation of the cervix; the second stage is from full cervical dilation until delivery of the baby; and the third stage begins with delivery of the baby and ends with delivery of the placenta. The first stage of labor is divided into two phases: the first is the latent phase, and the second is the active phase . The latent phase begins with the onset of labor and is characterized by regular, painful uterine contractions and a slow rate of cervical change. When the rate of cervical dilation is accelerated, latent labor ends and active labor begins. Labor onset is a retrospective diagnosis that is difficult to identify objectively. It is defined by the initiation of regular painful contractions of sufficient duration and intensity to result in cervical dilation or effacement. Women are frequently at home during this time; therefore the identification of labor onset depends on patient memory and the timing of contractions in relation to the cervical examination. The active phase of labor is defined as the period in which the greatest rate of cervical dilation occurs. Identification of the point at which labor transitions from the latent to the active phase will depend upon the frequency of cervical examinations and retrospective examination of labor progress. Historically, based upon Friedman’s seminal data on cervical dilation and labor progress from the 1950s and 1960s, active labor required 80% or more effacement and 4 cm or greater dilation of the cervix. He analyzed labor progress in 500 nulliparous and multiparous women and reported normative data that have been used for more than half a century to define our expectations of normal and abnormal labor.
Friedman revolutionized our understanding of labor because he was able to plot static observations of cervical dilation against time and successfully translate the dynamic process of labor into a sigmoid-shaped curve ( Fig. 12-13 ). Friedman’s data popularized the use of the labor graph, which first depicted only cervical dilation and was then later modified to include fetal descent. Four-centimeter cervical dilation marks the transition from the latent to the active phase because it corresponds to the flexion point on the averaged labor curve generated from a review of 500 individual labor curves in the original Friedman dataset. Rates of 1.5 and 1.2 cm dilation per hour in the active phase for multiparous and nulliparous women, respectively, represent the 5th percentile of normal. These data have led to the general concept that in active labor, a rate of dilation of at least 1 cm per hour should occur.
More recent analysis of contemporary labor from several studies challenges our understanding of the cervical dilation at which active labor occurs and suggests that the transition from the latent phase to the active phase of labor is a more gradual process. An analysis of labor curves for 1699 multiparous and nulliparous women who presented in spontaneous labor at term and underwent a vaginal delivery determined that only half of the women with a cervical dilation of 4 cm were in the active phase. By 5 cm of cervical dilation, 75% of the women were in the active phase, and by 6 cm cervical dilation, 89% of the women were in the active phase. Zhang and colleagues reviewed data from the National Collaborative Perinatal Project, a historic cohort of 26,838 term parturients in spontaneous labor from 1959 through 1966. This study used a repeated measures analysis to construct labor curves for parturients whose intrapartum management was similar to those studied by Friedman in the 1950s. The cesarean delivery rate was 5.6%, and only 20% of nulliparas and 12% of multiparas received oxytocin for labor augmentation. This study determined that labor progress in nulliparous women who ultimately had a vaginal delivery is in fact slower than previously reported until 6 cm of cervical dilation. Specifically, most nulliparous women were not in active labor until approximately 5 to 6 cm of cervical dilation, and the slope of labor progress did not increase until after 6 cm. These findings were confirmed in an analysis of contemporary data collected prospectively by the Consortium on Safe Labor, which enrolled and followed 62,415 singleton term parturients who presented in spontaneous labor at 19 institutions from 2002 through 2007. This dataset included a greater percentage of women with oxytocin augmentation (45% to 47%) and epidural analgesia (71% to 84%) compared with those studied by Friedman in the 1950s. Zhang and colleagues reported the median and 95th percentile of time to progress from one centimeter to the next and confirmed that labor may take more than 6 hours to progress from 4 to 5 cm and more than 3 hours to progress from 5 to 6 cm regardless of parity ( Table 12-2 ). Multiparas had a faster rate of cervical dilation compared with nulliparas only after 6 cm of cervical dilation had been reached. These data suggest that it would be more appropriate to utilize a threshold of 6 cm cervical dilation to define active phase labor onset and that the rate of cervical dilation for nulliparas at the 95th percentile of normal may be greater than the 1 cm per hour previously expected. These are important findings that suggest clinicians using the Friedman dataset to determine the threshold for active labor may be diagnosing active phase arrest prematurely, which could result in unnecessary cesarean deliveries (see Chapter 13 ).
| CERVICAL DILATION (cm) | PARITY 0 * | PARITY 1 | PARITY ≥2 |
|---|---|---|---|
| 3-4 | 1.8 (8.1) | – | – |
| 4-5 | 1.3 (6.4) | 1.4 ( 7.3) | 1.4 (7.0) |
| 5-6 | 0.8 (3.2) | 0.8 (3.4) | 0.8 (3.4) |
| 6-7 | 0.6 (2.2) | 0.5 (1.9) | 0.5 (1.8) |
| 7-8 | 0.5 (1.6) | 0.4 (1.3) | 0.4 (1.2) |
| 8-9 | 0.5 (1.2) | 0.3 (1.0) | 0.3 (0.9) |
| 9-10 | 0.5 (1.8) | 0.3 (0.9) | 0.3 (0.8) |
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