Abnormal Labor

To define abnormal labor, a definition of normal labor must be understood and accepted. Normal labor is defined as uterine contractions that result in progressive dilation and effacement of the cervix. By following thousands of labors resulting in uncomplicated vaginal deliveries, time limits and progress milestones have been identified that define normal labor. Failure to meet these milestones defines abnormal labor, which suggests an increased risk of an unfavorable outcome. Thus, abnormal labor alerts the obstetrician to consider alternative methods for a successful delivery that minimize risks to both the mother and the infant.

Dystocia of labor is defined as difficult labor or abnormally slow progress of labor. Other terms that are often used interchangeably with dystocia are dysfunctional labor, failure to progress (lack of progressive cervical dilatation or lack of descent), and cephalopelvic disproportion (CPD).

Friedman's original research in 1955 defined the following three stages of labor[1] :

1. The first stage starts with uterine contractions leading to complete cervical dilation and is divided into latent and active phases. In the latent phase, irregular uterine contractions occur with slow and gradual cervical effacement and dilation. The active phase is demonstrated by an increased rate of cervical dilation and fetal descent. The active phase usually starts at 3-4 cm cervical dilation and is subdivided into the acceleration, maximum slope, and deceleration phases.

2. The second stage of labor is defined as complete dilation of the cervix to the delivery of the infant.

3. The third stage of labor involves delivery of the placenta.

See images below for the normal labor curves of both nulliparas and multiparas. The following table shows abnormal labor indicators.

Table. Abnormal Labor Indicators (Open Table in a new window)

 

Abnormal labor constitutes any findings that fall outside the accepted normal labor curve. However, the authors hesitate to apply the diagnosis of abnormal labor during the latent phase because it is easy to confuse prodromal contractions for latent labor. In addition, the original labor curve, as defined by Friedman, may not be completely applicable today.[2, 3, 4, 5]

Contemporary practice with supporting data suggest that the duration of labor appears longer today than in the past. For both nulliparous and multiparous women, labor may take longer than 6 hours to progress from 4 cm to 5 cm and longer than 3 hours to progress from 5 cm to 6 cm of dilation. Cervical dilation of 6 cm appears to be a better landmark for the start of the active phase. The 95th percentile for duration of the second stage in a nulliparous woman with conduction anesthesia is closer to 4 hours.[6] Note the charts below.

First stage of labor

As stated above, first stage of labor is divided into latent and active phases. According to Friedman et al, latent stage considered to be prolonged if takes >20 hours for nulliparous women and >14 hours for multiparous women. However, prolonged latent phase does not usually lead to any clinically significant adverse events for mother or the infant. Therefore, diagnosis of abnormal labor during the latent phase is uncommon and is not relevant for clinical practice.

Around the time uterine contractions cause the cervix to become 3-4 cm dilated, the patient usually enters the active phase of the first stage of labor, according to the traditional definition. Abnormalities of cervical dilation (protracted dilation and arrest of dilation) as well as descent abnormalities (protracted descent and arrest of descent) as described historically are outlined in the Table above.

Both American College of Obstetrics and Gynecology (ACOG) and the Consortium on Safe Labor have proposed extending the minimum period before diagnosing active-phase arrest. The Consortium on Safe Labor defines 6 hours as the 95th percentile of time to go from 4 cm to 5 cm dilation, with the active phase defined as beginning at 6 cm (instead of 4 cm). According to this study, the 95th percentile of rate of dilation in active phase is 0.5 cm/hr to 0.7 cm/hr for nulliparous women and from 0.5 cm/h to 1.3cm/hr for multiparous women.[7]   ACOG has also stated that extending the time from 2 to 4 hours with oxytocin augmentation appears effective. Irrespective of the duration, maternal and fetal well-being status must be confirmed. In another study it was found that extending oxytocin augmentation for an additional 4 hours, up to 8 hours total, in patients who were dilated at least 3cm and had unsatisfactory progress resulted in a greater number of vaginal deliveries (38% delivered vaginally) without any evidence of fetal compromise.[8]

According to the most recent evidence, arrest of labor in the first stage should be defined as more than or equal to 6cm dilation with ruptured membranes and one of the following: 4 hours or more of adequate contractions (>200 MVU) or 6 hours or more of inadequate contractions and no cervical change.[9]

The maternal risk of a first stage greater than the 95th percentile (>30 h) is associated with a higher cesarean delivery rate (adjusted odds ratio [aOR]: 2.28) and chorioamnionitis (aOR: 1.58). The neonatal risk is associated with a higher incidence of neonatal ICU admissions in the absence of any other of the major morbidities (aOR: 1.53).[6] These results were again confirmed by another study, which established that prolonged first stage of labor lead to increased risks of a prolonged second stage, maternal fever, shoulder dystocia, and admission to a level 2-3 nursery[10]  

Second stage of labor

The Consortium on Safe Labor also addressed the 95th percentile for the second stage for nulliparous women; it was 2.8 hours (168 min) without regional anesthesia and 3.6 hours (216 min) with regional anesthesia. For multiparous women, the 95th percentiles for second-stage duration with and without regional anesthesia remained around 2 hours and 1 hour, respectively.[6, 7] However, other studies demonstrate the risks of both maternal and perinatal adverse outcomes rising with increased duration of the second stage, particularly for durations longer than 3 hours in nulliparous women and 2 hours in multiparous women.[11] One study found that if nulliparous women delivered after prolonged second stage, they were twice as likely to have operative vaginal delivery, three times as likely to develop chorioamnionitis, have higher odds of having episiotomy and 3rd or 4th degree lacerations, and one day longer median hospital stay.[12] .Thus, careful clinical assessment of fetal and maternal well-being must be confirmed when extending the duration of the first and second stages of labor.

Third stage of labor

The third stage of labor is the time from delivery of the infant to placental delivery. Historically, the duration of a normal third stage of labor was defined as less than 30min. This threshold was established by the study conducted by Combs et al. in 1991, in which 75% of placentas were delivered by 10 minutes and maternal morbidity and mortality increased if the third stage was increased beyond 30 min. However, a recently published article by Frolova et al, puts this definition into a question. In this study, the researchers found that in a modern cohort, 90% of placentas are delivered within 10 minutes and the risk of postpartum hemorrhage almost doubles by the time the duration of third stage reaches twenty minutes.[13]  Therefore, the traditional definition of prolonged third stage being >30 min, may be outdated and in need of revision.

 

In general, abnormal labor is the result of problems with one of the following three P' s:

• Passenger (infant size, fetal presentation [occiput anterior, posterior, or transverse])

• Pelvis or passage (size, shape, and adequacy of the pelvis)

• Power (uterine contractility)

A prolonged latent phase may result from oversedation or from entering labor early with a thickened or uneffaced cervix. It may be misdiagnosed in the face of frequent prodromal contractions. Protraction of active labor is more easily diagnosed and is dependent upon the 3 P' s.

The first P, the passenger, may produce abnormal labor because of the infant's size (eg, macrosomia) or from malpresentation.

The second P, the pelvis, can cause abnormal labor because its contours may be too small or narrow to allow passage of the infant. Both the passenger and pelvis cause abnormal labor by a mechanical obstruction, referred to as mechanical dystocia.

With the third P, the power component, the frequency of uterine contraction may be adequate, but the intensity may be inadequate. Disruption of communication between adjacent segments of the uterus may also exist, resulting from surgical scarring, fibroids, or other conduction disruption. Whatever the cause, the contraction pattern fails to result in cervical effacement and dilation. This is called functional dystocia. Uterine contractile force can be quantified by the use of an intra-uterine pressure catheter. Use of this device allows for direct measurement and calculation of uterine contractility per each contraction and is reported in Montevideo units (MVUs). For uterine contractile force to be considered adequate, the force produced must exceed 200 MVUs during a 10-minute contraction period. Arrest disorders cannot be properly diagnosed until the patient is in the active phase and had no cervical change for 2 or more hours with the contraction pattern exceeding 200 MVUs. Uterine contractions must be considered adequate to correctly diagnose arrest of dilation.[14]

Prolonged latent phase

The latent phase of labor is defined as the period of time starting with the onset of regular uterine contractions and ending with the onset of the active phase (usually 3-4 cm cervical dilation).

A prolonged latent phase is defined as exceeding 20 hours in patients who are nulliparas or 14 hours in patients who are multiparas.

The most common reason for prolonged latent phase is entering labor without substantial cervical effacement.

Power

Power is defined as uterine contractility multiplied by the frequency of contractions.

Montevideo units (MVUs) refer to the strength of contractions in millimeters of mercury multiplied by the frequency per 10 minutes as measured by intrauterine pressure transducer.

The uterine contraction pattern should repeat every 2-3 minutes.

The uterine contractile force produced must exceed 200 MVUs/10 min for active labor to be considered adequate. For example, 3 contractions in 10 minutes that each reach a peak of 60 mm Hg are 60 X 3 = 180 MVUs.

An arrest disorder of labor cannot be diagnosed until the patient is in the active phase and the contraction pattern exceeds 200 MVUs for 2 or more hours with no cervical change. Extending the minimum period of oxytocin augmentation for active-phase arrest from 2 up to 4 hours may be considered as long as fetal reassurance is noted with fetal heart rate monitoring.

Pelvis or the size of the passageway inhibiting delivery

The shape of the bony pelvis (eg, anthropoid or platypelloid) can result in abnormal labor.

A patient who is extremely short or obese, or who has had prior severe trauma to the bony pelvis, may also be at increased risk of abnormal labor.

Size and/or presentation of the infant

Abnormal labor could also be secondary to the passenger, the size of the infant, and/or the presentation of the infant. In addition to problems caused by the differential in size between the fetal head and the maternal bony pelvis, the fetal presentation may include asynclitism or head extension. Asynclitism is malposition of the fetal head within the pelvis, which compromises the narrowest diameter through the pelvis. Fetal macrosomia and other anomalies (including hydrocephalus, encephalocele, fetal goiter, cystic hygroma, hydrops, or any other abnormality that increases the size of the infant) are likely to cause deviation from the normal labor curve.

Other factors

Other factors include either a low-dose epidural or combined spinal-epidural anesthetics that minimize motor block and may contribute to a prolonged second stage. These have also been associated with an increase in oxytocin use and operative vaginal delivery. However, use of epidural for analgesia during labor does not result in a statistically significant increase in cesarean delivery.[15]  Intravenous oversedation has also been implicated as prolonging labor in both the latent and active phases.

United States statistics

Of all cephalic deliveries, 8-11% are complicated by an abnormal first stage of labor. Dystocia occurs in 12% of deliveries in women without a history of prior cesarean delivery. Dystocia may account for as many as 60% of cesarean deliveries.

The prognosis of subsequent pregnancies depends on the cause for abnormal labor. For example, if abnormal labor occurs from macrosomia, the next infant may not be macrosomic. However, if the abnormal labor was secondary to a contracted pelvis with a normal-sized or small infant, then the likelihood for a recurrence of abnormal labor is high.

In an attempt to determine whether increasing maternal age is more commonly associated with dystocia, a study by Treacy et al demonstrated that the incidences of oxytocin augmentation, prolonged labor, instrument delivery, and intrapartum cesarean delivery (including cesarean for dystocia) all increased significantly and progressively with increasing maternal age.[16]  This study used an established active management protocol, and oxytocin augmentation proved a generally effective intervention in all age categories. These findings have implications for the analysis of intervention rates by health care providers, particularly in developed countries where the proportion of older nulliparas is increasing.

A study by Zhu et al revealed that, with increasing interpregnancy intervals, the risk for labor dystocia increases.[17]  Both functional and mechanical dystocia were more prevalent in first births than in subsequent births. In singleton births to multiparous mothers, labor dystocia was associated with the interpregnancy interval in a dose-response fashion. Compared with an interpregnancy interval of less than 2 years, the adjusted odds ratios that was associated with interpregnancy intervals of 2-3, 4-5, 6-7, 8-9, and 10+ years were 1.06, 1.15, 1.25, 1.31, and 1.50, respectively, when controlled for other reproductive risk factors. Functional dystocia was associated more strongly with interpregnancy interval than mechanical dystocia.

Morbidity/mortality

Both maternal and fetal mortality and morbidity rates increase with abnormal labor. This is probably an effect-effect relationship rather than a cause-effect relationship. Nonetheless, identification of abnormal labor and initiation of appropriate actions to reduce the risks are matters of some urgency.

A meta-analysis by Pergialiotis et al found that a prolonged second stage of labor was associated with a higher risk of postpartum hemorrhage, chorioamnionitis, endometritis, postpartum fever, and obstetric anal sphincter injury. In addition, the risk of neonatal sepsis was higher, as was the likelihood of admission to the neonatal intensive care unit.[18]

Complications

Maternal infection is a risk, especially when rupture of membranes occurs for more than 18 hours. Administer antibiotics for signs and symptoms of chorioamnionitis.

Fetal compromise can occur from the inability to tolerate labor (eg, uterine hyperstimulation) or infection, and it must be closely evaluated. Fetal heart monitoring often reveals signs of compromise with decelerations, and fetal scalp pH is an option when indicated.

Probably the most common complication of the medical induction of labor is hyperstimulation of the uterus. If unrecognized and untreated, excessive stimulation of the uterus can result in fetal compromise, cord compression, and uteroplacental insufficiency. Uterine rupture, postpartum uterine atony, and postpartum hemorrhage may occur and can be life-threatening complications requiring emergent action.

Allen et al found that increased duration of the second stage of labor—in particular, duration longer than 3 hours in nulliparous women and longer than 2 hours in multiparous women—increases the risk of both maternal and perinatal adverse outcomes. In their population-based cohort study in 121,517 women (52% nulliparous), women with a prolonged second stage were at increased risk for obstetric trauma, postpartum hemorrhage, puerperal febrile morbidity, and composite maternal morbidity, while their infants were at increased risk for low 5-minute Apgar score (see the Apgar Score calculator), birth depression, admission to the neonatal intensive care unit, and composite perinatal morbidity. Method of delivery modified the effect of duration of second stage among nulliparous women only.[11]

Nerve injury is more common in nulliparous women and is associated with long labor, fetal macrosomia, and certain positions that women assume during labor. Peripheral nerve injuries typically manifest as weakness or numbness in the distribution of the affected nerve(s), and most authors describe an uncomplicated resolution of the neuropathy within approximately 6 months. The development of a complex regional pain syndrome has also been described.[19]

The patient must be aware of all risks involved with labor, including the potential for emergent cesarean delivery if the fetus appears compromised. Furthermore, she should be kept informed of her status throughout the labor course, especially if a change in management is anticipated. Counsel patients early in pregnancy that maternal weight gain correlates with fetal weight gain, and excessive gain and prepregnancy obesity are risk factors for abnormal labor.

For patient education resources, see Women's Health Center and Pregnancy Center, as well as Labor Signs.

Evaluate every pregnant patient who presents with contractions in the labor and delivery unit. Any patient in labor is at risk for abnormal labor regardless of the number of previous pregnancies or the seemingly adequate dimensions of the pelvis. Plot the progress of any patient in labor, and evaluate it on a labor curve (see images below).

Upon admission to the labor and delivery unit, determine and document clinical findings.

Clinical pelvimetry, which is best performed at the first prenatal care visit, is important in order to assess the pelvic type (eg, android, gynecoid, platypelloid, anthropoid). Evaluate the position of the fetal head in early labor because caput and moulding complicate correct assessment as labor progresses. Establish and document an estimated fetal weight. Monitor fetal heart rate and uterine contraction patterns to assess fetal well-being and adequacy of labor. Perform a cervical examination to determine whether the patient is in the latent or active phase of labor.

Addressing these issues allows for an assessment of the current phase of labor and anticipation of whether abnormal labor from any of the 3 P' s may be encountered.

No specific laboratory studies are used to assess abnormal labor.

Other tests

The simplest test used to evaluate abnormal labor is to plot the patient's labor progress (cervical dilation vs duration in hours) on a labor curve.

A second test used to address adequate labor is the review of the uterine contraction pattern by determining adequacy of contractions with use of an intrauterine pressure catheter.

Most importantly, the fetal heart tracing must be reassuring throughout the labor course.

Clinical pelvimetry, at a minimum, must address the angles of the spinous processes (convergent, divergent, straight), the bi-ischial diameter (>8 cm), the distance to the sacral promontory from the symphysis pubis (>12 cm), and the relation of the bony pelvis to the fetal head.

Clinical pelvimetry requires experience and deliberate attention to the question of pelvic adequacy. It cannot account for fetal size or strength/frequency of contractions, but, in experienced hands, it may reliably identify a pelvis as adequate, borderline, or contracted.

An estimate of the fetal weight must be documented in the hospital chart early in the labor course. If concern for macrosomia exists, this must be addressed with the patient and the labor/delivery team in order to anticipate and prepare for labor dystocia.

A prolonged latent phase (see Table in Background) is not indicative of dystocia in itself because this diagnosis cannot be made in the latent phase. Gabbe and colleagues state the following:[20]

For those in the latent phase, the treatment of choice is rest for several hours. During this interval, uterine activity, fetal status, and cervical effacement must be evaluated to determine if progress to the active phase has occurred. Approximately 85% of patients so treated progress to the active phase. Approximately 10% will cease to have contractions, and the diagnosis of false labor may be made. For the approximately 5% of patients in whom therapeutic rest fails and in patients for whom expeditious delivery is indicated, oxytocin infusion may be used.

Use of oxytocin for active management of labor is described in the Medication section.

Limited studies have shown improvement in dysfunctional labor with use of a beta-blocker. In cases of dysfunctional labor resulting from functional dystocia or an abnormal uterine contractility pattern and in which oxytocin implementation has not improved the outcome, a beta-blocker may be considered. Low-dose administration of intravenous propranolol in abnormal labor augmented with oxytocin reduced the need for cesarean delivery, particularly among patients with inadequate uterine contractility.[21, 22]

Anecdotal reports have stated that simply repositioning the patient frequently relieves a seemingly obstructed labor. Although not studied rigorously, there appears to be little harm in this maneuver. In theory, it may unseat an asynclitic or malrotated presenting part and allow it to engage in the pelvis more effectively.

Some studies looked at the use of the “peanut ball”, which is an exercise ball shaped like a peanut, and found that use of it during labor significantly decreased the length of first stage of labor in nulliparous women. [23] Another study on the same topic found that both first and second stages of labor were shortened by 29 min and 11 min respectively, and cesarean delivery rate was significantly reduced.[24]   Overall, given the low risk of such intervention, it appears to be worth trying in prolonged labor, particularly in nulliparous women. 

Induction of labor

In a large cohort of nulliparous women who delivered singleton live births at 39-42 weeks, Cheng et al observed that induction of labor was not associated with an increased risk of cesarean delivery compared with delivery at a later gestational age.[25] Additionally, the risk of labor dystocia for women who were induced at 39 weeks (5.93%) was lower than for those expectantly managed and delivered later (6.71%; aOR, 0.88). Labor dystocia was also less likely for women who had induction at 40 weeks compared with delivery later. Additionally, no difference in risk of operative vaginal delivery, including forceps or vacuum-assisted vaginal delivery, was reported.

While these data support that induction may provide improved perinatal outcomes, without impacting labor dystocia or increasing cesarean delivery rate, the authors caution generalized implementation and recommend future large prospective, randomized, clinical trials to further assess the potential benefit in low-risk populations.[25]

Amniotomy is often used and has become an accepted practice once the patient has reached the active phase of labor, although it has not been shown to result in shorter labor.[26] This practice is not recommended in the latent phase of labor because it may only serve to increase the risk of intrauterine infection or cord prolapse.

In the case of the prolonged second stage due to fetal malposition, the operator may attempt manual rotation of the fetal occiput. Studies have shown significant decrease in the rate of cesarean delivery with no increase in adverse neonatal outcomes with this maneuver[27, 9, 28]

If one of the arrest or protraction disorders is identified and fails to respond to conservative measures, or if the fetal heart pattern is nonreassuring, expedient delivery is justified; this includes operative vaginal delivery (if appropriate) or cesarean delivery as indicated. Operative delivery with use of forceps or vacuum must be performed by an experienced provider. One should be aware of the increased associations for shoulder dystocia and neonatal injury with operative vaginal delivery in the setting of abnormal labor.[29, 30]

Diet

Most institutions have standing orders that patients in labor have nothing by mouth as a precaution should the need for an emergent cesarean delivery arise.

Some institutions permit ice chips, and others permit a clear liquid diet.

If patients have been carefully selected as low risk for labor obstruction, a regular diet may be ordered.

Pregnant women have delayed gastric emptying, and aspiration is a very serious concern in the event of an anesthetic induction.

Activity

For patients in labor, remaining active and mobile while in the latent and early active phase is best. However, once rupture of membranes has occurred or signs of fetal nonreassurance exist, then bed rest and continuous fetal monitoring is appropriate.

Some clinicians allow ambulation throughout labor as long as the fetal head is well applied (minimizing risk of cord prolapse) and evidence of fetal well-being exists (monitoring for 20 min/h without signs of fetal compromise).

A protocol called active management of labor can be applied to nulliparous women with singleton cephalic presentations at term. This method involves the use of high-dose oxytocin, with a starting rate of 6 mU/min and increasing by 6 mU/min every 15 min to a maximum of 42 mU/min. This is in contrast to the most common oxytocin algorithms used in hospitals in the United States, which typically involve increasing the oxytocin dose 1-2 mU/min every 30-40 minutes. Either way, the goal is to have no more than 7 uterine contractions per 15 min. Under this higher dose protocol, cesarean delivery is performed if vaginal delivery has not occurred or is not imminent 12 hours after admission or for fetal compromise. Initially, cesarean delivery rates were quoted at 4.8%, but it has since doubled, which is attributed to widespread use of epidural anesthesia. Other studies using the active management protocol describe cesarean delivery rates similar to that of the low-dose protocol. Randomized clinical trials have shown that the high-dose oxytocin regimens result in shorter labors than low-dose regimens without adverse effects for the fetus.[31]

A study by Brüggemann et al in term nulliparous women found that the rate of cesarean section was highest in women with labor dystocia that required initiation of oxytocin at 5 cm or less of cervical dilatation. The researchers suggest that oxytocin augmentation before 6 cm of cervical dilatation may fail to reduce the need for cesarean section.[32]

Dinoprostone and misoprostol are prostaglandin analogs used to stimulate cervical dilation and uterine contractions; they are pharmacologic alternatives to using laminaria or placing a Foley bulb in the cervix. Using prostaglandin analogs with a scarred uterus (eg, from prior cesarean or myomectomy) for labor induction is absolutely contraindicated due to the significant risk for uterine rupture.

A randomized clinical trial testing the safety and efficacy of prostaglandin E2 (PgE2) as a treatment for dystocia in spontaneous labor revealed that a single 1-mg dose of PgE2 vaginal gel is more effective than placebo in resolving dystocia without increasing uterine hyperstimulation, but it may be associated with an increase in the incidence of second stage cesarean delivery.[33]

Class Summary

Oxytocin is the only US Food and Drug Administration (FDA)–approved medication recommended for labor augmentation. Other options include dinoprostone and misoprostol.

Oxytocin (Pitocin)

Produces rhythmic uterine contractions and can stimulate the gravid uterus. Has vasopressive and antidiuretic effects. Can also control postpartum bleeding or hemorrhage. Has a half-life of 3-5 min, and reaches steady state in approximately 40 min.

Class Summary

Another option for abnormal labor secondary to inadequate uterine contractility is a beta-blocker.

Propranolol (Inderal)

Nonselective beta-adrenergic receptor blocker.