Fewer than 10% of pregnancies end on the expected date of delivery (EDD), although most births occur within 7 days of the EDD. In the United States, approximately 10% of births occur preterm (before 37 weeks’ gestation) (see Chapter 33 ), and approximately 5% to 7% of pregnancies remain undelivered at 42 weeks’ gestation ( postterm ). These rates are lower for carefully dated pregnancies.

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, the number of myometrial 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. Preterm and postterm deliveries both constitute important obstetric problems. Better understanding of the mechanism of the onset of human labor may lead to new approaches to preventing the preterm and postterm onset of parturition.

The uterus, which is a smooth muscle organ, contracts throughout gestation with variable frequency. In some women, the uterus remains relatively quiescent until the abrupt onset of labor. In others, the uterus contracts several times per hour for days without causing pain or even a clear perception of uterine contractions. The parturient verifies the onset of labor when she perceives regular, uncomfortable uterine contractions.

During labor, the frequency, duration, and intensity of uterine contractions increase. During the latent phase of the first stage of labor, the contractions may occur every 5 to 7 minutes, last 30 to 40 seconds, and develop intrauterine pressures (intensity) of 20 to 30 mm Hg above basal tone (10 to 15 mm Hg). In the active phase, contractions typically occur every 2 to 3 minutes, last 50 to 70 seconds, and are 40 to 60 mm Hg in intensity. This higher intensity reflects a more widespread propagation of the contractions, with the recruitment of more myometrial cells.

Retraction accompanies contraction as the myometrial cells shorten. The walls of the upper, contractile portion of the uterus thicken. Cervical dilation and effacement reflect the traction placed on the cervix by the contracting uterus. The passive lower uterine segment enlarges and becomes thinner as cervical tissue is pulled over the fetal presenting part by traction from the upper portion of the uterus. At the end of the first stage of labor, there is complete cervical dilation. If there is no mechanical obstruction, additional uterine contractions force the fetus to descend through the birth canal. When this occurs, the parturient often perceives an urge to defecate, reflecting pressure on the rectum. The parturient’s expulsive efforts add to the force of uterine contractions to hasten descent and shorten the second stage of labor.

The fetus is ideally of a size and conformation that there is no mechanical mismatch with the bony pelvis. The type and size of the pelvis constitute important predictors of the success of vaginal delivery. Rarely, an ovarian or uterine tumor (e.g., leiomyoma), cervical cancer, or a vaginal septum may impede passage of the fetus through the birth canal.

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 19.1 ).

The most common pelvic type, the gynecoid pelvis, is particularly well suited to accommodate the flexed fetal head within the bony pelvis. The inlet is round or oval, with the transverse diameter only slightly greater than the anteroposterior diameter. The pelvic sidewalls are straight and do not converge, the ischial spines are not prominent, the sacrum is hollow, and the subpubic arch is wide. The absence of prominent ischial spines is an important feature because the distance between them—the transverse diameter of the mid-pelvis—is the narrowest pelvic dimension. The other pelvic types are thought to be less favorable for vaginal delivery.

Radiographic pelvimetry was historically thought to provide more information regarding pelvic dimensions and features than can be obtained by clinical pelvimetry alone. However, large metaanalyses have shown no utility in its use for clinical management because of its poor ability to predict a successful vaginal birth and its association with an increased rate of cesarean delivery. Computed tomography and magnetic resonance imaging are associated with less or no ionizing radiation exposure, but data are limited on their utility for predicting a successful vaginal delivery.

In the absence of a history of pelvic fracture or rare musculoskeletal disease (e.g., a dwarfing condition), there are few circumstances in which the 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, molding is sufficient (i.e., overlapping of the fetal unfused skull bones), and contractions are adequate (>200 Montevideo units).

Fetal size and the relationship of the fetus to the maternal pelvis can affect labor progress. The lie of the fetus (the relationship of the long axis of the fetus to the long axis of the mother) can be transverse, oblique, or longitudinal. In the first two, vaginal delivery is impossible unless the fetus is very immature.

The presentation denotes that portion of the fetus overlying the pelvic inlet. The presentation may be cephalic, breech, or shoulder. Cephalic presentations are further subdivided into vertex, brow, or face presentations according to the degree of flexion of the neck. In more than 95% of labors at term, the presentation is cephalic and the fetal head is well flexed (i.e., vertex presentation).

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 are listed in Box 19.1 . Other markers for position are the sacrum for breech presentation, the mentum for face presentation, and the acromion for shoulder presentation. (See Chapter 34 for a discussion of nonvertex presentations.)

The mechanism of labor refers to the changes in fetal conformation and position (cardinal movements) ( Box 19.2 ) that occur during descent through the birth canal during the late first 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. Obstetricians judge that engagement has occurred when the leading bony point of the fetal head is palpable at the level of the ischial spines. This is a reasonable proxy 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 ( Fig. 19.1 ). 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.

Stations of the fetal head.

Redrawn from Zlatnik FJ. Normal labor and delivery and its conduct. In: Scott JR, DiSaia PJ, Hammond CB, Spellacy WN, eds. Danforth’s Obstetrics and Gynecology . 7th ed. JB Lippincott; 1994:116.

The second cardinal movement is descent , although descent occurs throughout the birth process. The third cardinal movement is flexion . A very small fetus can typically descend through the maternal pelvis without increased flexion. However, under the usual circumstances at term, the force from uterine contractions from above the fetus and resistance from below enhance flexion of the occiput ( Fig. 19.2 ).

(A) Relation of the head to the vertebral column before flexion. (B) Relation of the head to the vertebral column after flexion.

Redrawn from Zlatnik FJ. Normal labor and delivery and its conduct. In: Scott JR, DiSaia PJ, Hammond CB, Spellacy WN, eds. Danforth’s Obstetrics and Gynecology . 6th ed. JB Lippincott; 1990:174.

The fourth cardinal movement is internal rotation . The fetus meets the narrowest pelvic dimension, the transverse diameter between the ischial spines, at the level of the mid-pelvis. Because the BPD of the fetal head is slightly smaller than the suboccipitobregmatic diameter, the vertex typically negotiates the mid-pelvis with the sagittal suture in an anteroposterior direction. Internal rotation describes the change in the position of the vertex from occiput transverse or oblique to anteroposterior. The occiput tends to rotate to the roomiest part of the pelvis; thus, in gynecoid pelvises, the fetus is delivered in an occiput anterior position.

The next cardinal movement is extension , which occurs as the fetal head delivers ( Fig. 19.3 ). Subsequently, the occiput rotates to the side (external rotation) as the shoulders pass through the mid-pelvis 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.

Vertex Presentations. (A) Occiput anterior position. (B) Occiput posterior position.

Redrawn from Zlatnik FJ. Normal labor and delivery and its conduct. In: Scott JR, DiSaia PJ, Hammond CB, Spellacy WN, eds. Danforth’s Obstetrics and Gynecology . 6th ed. JB Lippincott; 1990:174.

Abnormalities of the pelvis affect the mechanism of labor in specific ways. In an anthropoid pelvis, where the anteroposterior diameter of the pelvic inlet exceeds the transverse diameter, internal rotation to the occiput posterior position rather than the occiput anterior position often occurs. 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 Fig. 19.3 ).

In platypelloid pelvises, 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.

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 (chorioamnion); or some other reason? If the presumptive diagnosis is labor, is the fetus at term?

The time of the onset of labor and the 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 the latent or active phase of the first stage of labor. In patients with vaginal bleeding, manual examination of the cervix is deferred until after ultrasonography, unless placenta previa has been previously ruled out, to avoid exacerbation of bleeding. To decrease the risk of infection, cervical examination may also be deferred in patients with prelabor rupture of membranes.

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 (see Chapter 8 ).

The maternal vital signs and FHR are recorded periodically. In some obstetric services, continuous electronic FHR monitoring is used universally; with other services, FHR is monitored via intermittent auscultation. In low-risk patients, it is acceptable to record 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. During latent labor, the patient may ambulate or assume any position of comfort on the labor bed or in a chair. During active labor, some women choose to lie down. Evidence supports that ambulation should be recommended in the first stage of labor for women without neuraxial analgesia; for those with neuraxial analgesia, either ambulating or not is acceptable. For women with low-risk pregnancies, choices concerning analgesia or anesthesia are made according to the patient’s wishes. For some women with maternal, obstetric, or fetal comorbidities, clinicians may suggest neuraxial analgesia to decrease the maternal stress of labor or facilitate operative vaginal delivery (see Chapter 24 ).

During labor, those providing obstetric care must focus on the following two critical questions:

• 1.

  Is the fetus tolerating labor in a satisfactory fashion or is there evidence of fetal compromise (see Chapter 8 )?

• 2.

  What is the progress of labor?

One of the central tasks of those providing intrapartum care is to determine whether labor is progressing, and if not, to determine the significance of the delay and what the response should be. It is important to note that there is significant variation in the “normal” progress of labor; different definitions are used globally depending on the population. Many methods of measuring the progress of labor have been proposed, but there is no widely accepted definition of the “normal” progress of labor. A general overview of methods of labor progression assessment is detailed below.

A generation of obstetricians is indebted to Emanuel Friedman, whose landmark studies of labor provided a foundational 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, delineated the difference between latent and active phase, and offered guidance for the statistical limits of normal. Based on Friedman’s labor curve, Peisner and Rosen 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.

Parity, referring to previous pregnancies of at least 20 weeks’ gestation, is an important determinant of labor length. For example, a pregnant woman who is gravida 2, para 1 is pregnant for the second time, and her first pregnancy resulted in delivery after 20 weeks’ gestation. Fourteen hours is the suggested limit of the latent phase in the parous woman. 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 Fig. 19.4 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.

The Friedman Curve.

From Friedman EA. Patterns of labor as indicators of risk. Clin Obstet Gynecol . 1973;16:172–183.

Studies published in 2002 and 2010 using data from a large database of women who gave birth in the United States reported slower rates of cervical dilation and engendered an ongoing transition toward the use of the Zhang “contemporary labor curve.” These newer data have been adopted into new labor management practice guidelines. These labor curves reveal that cervical dilation is particularly slow prior to 6-cm cervical dilation 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 and should be used to define the active phase for labor management. Furthermore, in the active phase, the 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 to the maternity unit) by parity are shown in Table 19.2 . Nulliparous women have a slower cumulative rate of cervical dilation overall. However, before 6 cm of dilation, the time intervals required to dilate 1 cm are similar between nulliparous and parous women. While these “contemporary labor curves” have been emphasized in new practice guidelines, it is important to note the ongoing need for prospective evaluation of these new criteria, as comparisons of labor managed using the Friedman and Zhang labor curves have yielded mixed results. Furthermore, the routine use of a partogram (labor curve) to monitor labor progress (regardless of the labor curve utilized) is no longer recommended.

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 with an unfavorable (long, closed, firm) cervix. The Bishop score ( Table 19.3 ) helps quantitate the favorability of the cervix; the higher the score, the shorter the labor and the less likelihood of labor failure. This finding is not surprising, because a high Bishop score indicates that the cervix is ready to dilate with uterine contractions (i.e., the cervix will soon enter the active phase). By contrast, a low score suggests that many hours of uterine contractions may be needed to soften and efface the cervix.

Some women with a prolonged latent phase are not in true labor, but rather in “false labor” with Braxton-Hicks contractions; 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 may reappear in true labor.

Studies published in 1993 and 2011 reported an increased risk for cesarean delivery, chorioamnionitis, endometritis, excessive blood loss, depressed Apgar scores, and need for neonatal resuscitation with a prolonged latent phase. Primary dysfunctional labor and arrest of dilation during the active phase may indicate cephalopelvic disproportion. Whether using the historic Friedman or contemporary Zhang labor curve, it is clear that women who experience active-phase arrest of dilation are more likely to require cesarean delivery than women without arrest (regardless of definition) during the active phase.

In summary, the contemporary view of labor progress is that delays in the latent phase of the first stage of labor may be associated with fetopelvic disproportion or the need for cesarean delivery, but this is not universal, as some women may still achieve vaginal birth with prolonged labor without adverse outcomes. Delays in the active phase predict fetopelvic disproportion, although not with precision. Given current obstetric practice and fetal monitoring techniques, it is unclear whether first-stage labor abnormalities are intrinsically associated with neonatal depression at delivery.

Second stage of labor.

When the cervix has been completely retracted to form the lower uterine segment and is therefore not palpable on vaginal examination, full or complete dilation has been achieved, and the second stage of labor begins. Strong uterine contractions coupled with voluntary expulsive efforts by the parturient cause the fetal presenting part to descend through the pelvis, resulting in delivery. At complete cervical dilation, there is frequently an increase in bloody vaginal show, the parturient may vomit, and she may report rectal pressure. This sensation of needing to “bear down” encourages strong Valsalva maneuvers during uterine contractions. The duration of the second stage of labor is influenced by parity and the presence or absence of epidural analgesia. In the Zhang et al. study (Consortium on Safe Labor), the 95th percentile threshold for the duration of the second stage of labor was approximately 1 hour longer in parturients who received epidural analgesia compared with those who did not.

The contemporary obstetrician, although less concerned about the elapsed time during the second stage of labor than were obstetricians historically, continues to balance the risks of prematurely performing a cesarean delivery with the risks of adverse outcomes associated with a prolonged second stage. While studies have shown that a prolonged second-stage of labor is associated with an increase in maternal obstetric trauma, postpartum hemorrhage, infection, neonatal depression, and admission to the neonatal intensive care unit, particularly in the setting of difficult operative vaginal delivery, the contemporary view is that the second stage of labor does not need to be terminated at an arbitrary time provided that progress in descent continues and the FHR pattern is reassuring. Allowing a longer second-stage duration is an acceptable practice to decrease the rate of unnecessary cesarean deliveries.

If the parturient is allowed to choose her own position during labor and delivery, she often chooses not to stay in one place. Without instruction from birth attendants, the parturient frequently chooses to walk or sit in a chair during latent labor. In the active phase of the first stage, however, she often returns to the labor bed. During the second stage of labor, some women assume the squatting position, whereas others, with their legs supported by the nurse and the father of the baby, assume a semisitting position. The goal is to achieve a position in which the parturient’s Valsalva efforts are most effective. The patient should avoid the supine position without left uterine displacement, which results in aortocaval compression. Aortocaval compression seems to be less severe with the patient in the semisitting position, and it is avoided altogether with the patient in the lateral position.

An episiotomy is a perineal incision extending either directly posterior from 6 o’clock (midline) or in a 45-degree angle to either side (mediolateral). The former causes less discomfort, is more anatomic, and is easier to repair than the latter. The mediolateral episiotomy’s advantage is that extension through the anal sphincter and rectal mucosa is less likely to occur, but its major disadvantage is that it may cause more bleeding or severe postpartum pain. In the past, as the fetal head distended the perineum shortly before delivery, an episiotomy was often performed to shorten labor and to protect against the subsequent development of uterine prolapse, cystocele, and rectocele.

The role of episiotomy in contemporary obstetrics is limited. Tears involving the anal sphincter (third degree) and rectal mucosa (fourth degree) are actually more common after midline episiotomy than if episiotomy is not performed. In the absence of an episiotomy, however, anterior periurethral lacerations are common. These rarely cause immediate problems, but data on long-term outcomes are lacking. Given the recognized association between midline episiotomies and third- and fourth-degree tears, the association of these tears with long-term morbidity, and the failure to observe any benefits to routine episiotomy, more restrictive use of this incision is now recommended. An episiotomy may still be indicated in some instances of operative vaginal delivery of a large infant with suspected fetal compromise, or to manage shoulder dystocia.

The timing of initiation of maternal pushing efforts has been understudied. Pushing immediately upon full cervical dilation or allowing for spontaneous fetal descent (delayed pushing) have both been commonly used, but neither has been the “gold standard.” A 2018 randomized trial in nulliparous patients with neuraxial analgesia compared immediate and delayed pushing; the timing of pushing effort did not affect the rate of vaginal delivery. There were no differences in neonatal outcomes or perineal lacerations; however, immediate pushing was associated with decreased chorioamnionitis and postpartum hemorrhage. Current American College of Obstetricians and Gynecologists (ACOG) guidelines recommend pushing commence when complete cervical dilation is achieved (immediate pushing).

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