Williams Manual of Pregnancy Complications, 23 ed.

CHAPTER 14. Abnormal Labor and Delivery

Dystocia literally means difficult labor and is characterized by abnormally slow progress of labor. As a generalization, abnormal labor is common whenever there is disproportion between the presenting part of the fetus and the birth canal. It is the consequence of four distinct abnormalities that may exist singly or in combination (Table 14-1).

TABLE 14-1. Potential Causes of Abnormal Labor

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Dystocia can result from several distinct abnormalities involving the cervix, uterus, fetus, maternal bony pelvis, or other obstructions in the birth canal. These abnormalities can be simplified into three categories: (1) abnormalities of the powers (uterine contractility and maternal expulsive effort), (2) abnormalities of the passage (the pelvis), or (3) abnormalities involving the passenger (the fetus). Common clinical findings in women with these labor abnormalities are summarized in Table 14-2.

TABLE 14-2. Common Clinical Findings in Women with Ineffective Labor

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NORMAL LABOR

Dystocia is very complex, and although its definition—abnormal progress in labor—seems simple, there is no consensus as to what “abnormal progress” means. A strict definition of labor—uterine contractions that bring about demonstrable effacement and dilatation of the cervix—does not always aid the clinician because the diagnosis is confirmed only by birth.

In the United States, admission for labor is frequently based on the extent of dilatation accompanied by painful contractions. When the woman presents with intact membranes, cervical dilatation of 3 to 4 cm or greater is presumed to be a reasonably reliable threshold for diagnosis of true labor. In this case, onset of labor commences with the time of admission. This presumptive method of diagnosing true labor obviates many of the uncertainties in diagnosing labor during earlier stages of cervical dilatation.

First Stage of Labor

A scientific approach was pursued by Friedman to describe the normal progress of labor. He described a characteristic labor curve shown in Figure 14-1. This graph shows both a preparatory (or latent phase) and a dilatation (or active phase) of the first stage of labor. Also shown is a pelvic division, which corresponds to the descent of the fetal head through the pelvis. Figure 14-2 shows the average dilatation curve for nulliparous women as described by Friedman.

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FIGURE 14-1 Longitudinal lie. Vertex presentation. A. Left occiput anterior (LOA). B. Left occiput posterior (LOP). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

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FIGURE 14-2 Composite of the average dilatation curve for nulliparous labor. The first stage is divided into a relatively flat latent phase and a rapidly progressive active phase. In the active phase, there are three identifiable component parts that include an acceleration phase, a phase of maximum slope, and a deceleration phase. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010. From Friedman EA: Labor: Clinical Evaluation and Management. 2nd ed. New York, NY: Appleton-Century-Crofts; 1978.)

Latent Phase

The onset of latent labor is defined according to Friedman as the point at which the mother perceives regular contractions. During this phase, orientation of uterine contractions takes place along with cervical softening and effacement. The latent phase is accompanied by progressive, albeit slow, cervical dilatation, and ends between 3- and 5-cm dilatation.

Factors that affect duration of the latent phase include excessive sedation or epidural analgesia, poor cervical condition (e.g., thick, uneffaced, or undilated), and false labor. Active interventions in this phase of labor, such as oxytocin stimulation or amniotomy, is discouraged as oftentimes, false labor is confused with the active phase of labor. Importantly, prolongation of the latent phase of labor has not been associated with adverse fetal outcomes.

Active Phase

The active phase of the first stage of labor is generally accepted to begin with a cervical dilatation of 3 to 4 cm or more, in the presence of uterine contractions. Friedman reported the minimal average rate of cervical change to be 1.2 cm/h in nulliparas and 1.5 cm/h in multiparas. Considerable variation in the duration of labor was seen, however, with lengths of labor up to 11.9 hours being normal in nulliparous women.

Table 14-3 shows abnormal labor patterns and methods of treatment. These include prolongation disorders (prolonged latent phase), and protraction and arrest disorders (prolonged active phase). Protraction disorders refer to slowed rates of cervical dilatation or fetal descent. Arrest disorders refer to complete cessation of either cervical dilatation or fetal descent, or both.

TABLE 14-3. Clinical Outcomes in Relation to the Duration of Second-Stage Labor

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Second Stage of Labor

This stage begins when cervical dilatation is complete and ends with fetal expulsion. Its median duration is 50 minutes for nulliparas and 20 minutes for multiparas, but it is highly variable. In a woman of higher parity with a previously dilated vagina and perineum, two or three expulsive efforts after full cervical dilatation may suffice to deliver the infant. Conversely, in a woman with a contracted pelvis or a large fetus, or with impaired expulsive efforts from epidural analgesia or intense sedation, the second stage may become abnormally long.

The accepted normal upper limit of the second stage of labor in nulliparous women is 2 hours and 1 hour in multiparous with an additional hour for each being allowed in the presence of epidural analgesia. Recently, it has been shown that infant outcomes are not adversely affected if the duration of the second stage exceeds these limits; however, the prospects for successful vaginal delivery are reduced.

Summary of Normal Labor

Labor is characterized by brevity, considerable biological variation, and less complexity than anticipated based on contemporary graphostatistical interpretations. Active labor can be reliably diagnosed when cervical dilatation is 3 cm or more in the presence of regular uterine contractions. Once this cervical dilatation threshold is reached, normal progress to delivery can be expected, depending on parity, in the ensuing 4 to 6 hours. Anticipated progress during a 1- to 2-hour second stage is governed by limits intended to ensure fetal safety. Finally, most women in spontaneous labor, regardless of parity and if left unaided, will deliver within approximately 10 hours after admission for spontaneous labor. When time breaches in normal labor boundaries are the only pregnancy complications, interventions other than cesarean delivery must be considered before resorting to this method of delivery for failure to progress. Insufficient uterine activity is a common and correctable cause of abnormal labor progress.

Not only can labor be too slow as described in this chapter, labor can also be abnormally rapid—precipitate—that is, extremely rapid. This is defined as labor that terminates in expulsion of the fetus in less than 3 hours.

CAUSES OF INADEQUATE LABOR

Uterine Dysfunction

Propulsion and expulsion of the fetus is brought about by contractions of the uterus, reinforced during the second stage by voluntary or involuntary muscular action of the abdominal wall—“pushing.” Either of these factors may be lacking in intensity and result in delayed or interrupted labor. Uterine dysfunction, characterized by infrequent low-intensity contractions, is common with significant fetopelvic disproportion because the uterus does not often self-destruct when faced with mechanical obstruction. Uterine dysfunction in any phase of cervical dilatation is characterized by lack of progress, for one of the prime characteristics of normal labor is its progression. The diagnosis of uterine dysfunction in the latent phase is difficult and sometimes can be made only in retrospect. One of the most common errors is to treat women for uterine dysfunction who are not yet in active labor.

There have been three significant advances in the treatment of uterine dysfunction: (1) realization that undue prolongation of labor may contribute to perinatal morbidity and mortality, (2) use of dilute intravenous infusion of oxytocin in the treatment of certain types of uterine dysfunction, and (3) more frequent use of cesarean delivery rather than difficult midforceps delivery when oxytocin fails or its use is inappropriate.

Fetopelvic Disproportion

This situation arises from diminished pelvic size, excessive fetal size, or, more usually, a combination of both. Any contraction of the pelvic diameters that diminishes the capacity of the pelvis can create dystocia during labor. There may be contractions of the pelvic inlet, the midpelvis, the pelvic outlet, or a generally contracted pelvis caused by combinations of these.

Contracted Pelvic Inlet

The pelvic inlet is considered to be contracted if its shortest anteroposterior diameter is less than 10.0 cm or if the greatest transverse diameter is less than 12.0 cm. The anteroposterior diameter of the pelvic inlet is approximated by manually measuring the diagonal conjugate, which is approximately 1.5 cm greater. Therefore, inlet contraction is defined as a diagonal conjugate of less than 11.5 cm.

Prior to labor, the fetal biparietal diameter has been shown to average from 9.5 to as much as 9.8 cm. Therefore, it might prove difficult or even impossible for some fetuses to pass through an inlet with an anteroposterior diameter of less than 10 cm. The incidence of difficult deliveries is increased when the antero-posterior diameter of the inlet is less than 10 cm.

Contracted Midpelvis

This situation is more common than inlet contraction and is frequently a cause of transverse arrest of the fetal head, which can potentially lead to difficult midforceps operation or to cesarean delivery. The obstetrical plane of the midpelvis extends from the inferior margin of the symphysis pubis, through the ischial spines, and touches the sacrum near the junction of the fourth and fifth vertebrae.

Average midpelvis measurements are as follows: transverse (interspinous), 10.5 cm; anteroposterior (from the lower border of the symphysis pubis to the junction of the fourth and fifth sacral vertebrae), 11.5 cm; and posterior sagittal (from the midpoint of the interspinous line to the same point on the sacrum), 5 cm. Although there is no precise manual method of measuring midpelvic dimensions, a suggestion of contraction can sometimes be inferred if the spines are prominent, the pelvic sidewalls converge, or the sacrosciatic notch is narrow.

Contracted Pelvic Outlet

This situation is usually defined as diminution of the interischial tuberous diameter to 8 cm or less. A contracted outlet may cause dystocia not so much by itself as through the often-associated midpelvic contraction. Outlet contraction without concomitant midplane contraction is rare.

Pelvic Fractures and Rare Contractures

Trauma from automobile collisions is the most common cause of pelvic fractures. With bilateral fractures of the pubic rami, compromise of birth-canal capacity by callus formation or malunion is common. A history of pelvic fracture warrants careful review of previous x-rays and possibly computed tomographic pelvimetry later in pregnancy.

Abnormal Presentation, Position, and Development of the Fetus

A variety of abnormalities involving the passenger (fetus) are related to abnormal labor.

Excessive Fetal Size

The greatest obstetrical concern with excessive fetal size is not only whether the fetal head will fail to traverse the pelvic passage but also whether the fetal shoulders will fit through the pelvic inlet or midpelvis. In certain cases, such as diabetic women with estimated fetal weights exceeding 4250 to 4500 g, planned cesarean delivery may be appropriate.

Face Presentation

With a face presentation, the head is hyperextended so that the occiput is in contact with the fetal back and the chin (mentum) is presenting. The fetus may present with the mentum anterior or posterior, relative to the maternal symphysis pubis; and with labor impeded in mentum posterior presentations. However, many mentum posterior presentations convert spontaneously to anterior presentations, even late in labor.

Brow Presentation

This variation is the rarest presentation (see Table 14-4) and is diagnosed when the portion of the fetal head between the orbital ridge and anterior fontanel presents at the pelvic inlet. A persistent brow presentation will typically not allow engagement and subsequent delivery. With a small fetus and large pelvis, labor is generally easy. However, labor is usually difficult with a larger fetus unless significant molding occurs or there is conversion to an occiput or face presentation. Prognosis for vaginal delivery is poor without this conversion.

TABLE 14-4. Fetal Presentation in 68,097 Singleton Pregnancies at Parkland Hospital, 1995-1999

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Transverse Lie

This presentation is uncommon (see Table 14-4) and occurs when the long axis of the fetus lies perpendicular to the mother. It is called oblique when the long axis forms an acute angle. Causes may include multiparity (lax abdominal wall), preterm fetus, placenta previa, uterine anomaly, excessive amnionic fluid, and contracted pelvis. Spontaneous delivery of a fully developed infant is impossible with a persistent transverse lie and can result in a ruptured uterus.

Prior to the onset of labor with membranes intact, external version is reasonable. This technique should probably be performed after 39 weeks since 80 percent of transverse lies will spontaneously convert to a longitudinal lie before then. Once labor is established, cesarean delivery should be performed in a timely fashion.

Compound Presentation

These uncommon presentations (see Table 14-4) are usually related to conditions that prevent occlusion of the pelvic inlet by the fetal head (e.g., preterm birth) such that an arm or leg presents along with the fetal head. Perinatal loss is increased due to preterm delivery, prolapsed cord, and traumatic obstetric procedures.

Persistent Occiput Posterior Position

Most often occiput posterior positions undergo spontaneous anterior rotation and uncomplicated delivery. Labor and delivery need not differ remarkably from that of a fetus in the occiput anterior position.

However, there are notable differences when persistent occiput posterior is compared with occiput anterior position. Labor tends to be longer and there is a higher incidence of operative intervention, including forceps and cesarean delivery, for fetuses in persistent occiput posterior position. Operative vaginal delivery of these fetuses is more difficult and more likely to result in perineal lacerations than for fetuses with occiput anterior presentations.

Persistent Occiput Transverse Position

In the absence of abnormal pelvic architecture, the occiput transverse position is usually transitory. If rotation ceases during the second stage of labor because of lack of uterine action, and in the absence of pelvic contraction, vaginal delivery can usually be accomplished in a number of ways. The occiput may be manually rotated anteriorly or posteriorly and forceps delivery carried out from either the anterior or posterior position. If failure of spontaneous rotation is caused by inadequate contractions without cephalopelvic disproportion, oxytocin may be infused. In the presence of an abnormal pelvis, cesarean delivery is required.

Hydrocephalus

Delivery of a fetus with a hydrocephalic head is problematic, and the size of the fetal head must usually be reduced if passage through the birth canal is to occur. In the absence of other severe anomalies, cesarean delivery is recommended in most cases.

Fetal Abdomen as a Cause of Dystocia

Enlargement of the fetal abdomen sufficient to cause dystocia is usually the result of a greatly distended bladder, ascites, or enlargement of the kidneys or liver. Occasionally, the edematous fetal abdomen may attain such proportions that spontaneous delivery is impossible.

DIAGNOSIS OF INADEQUATE LABOR

Active-Phase Disorders

According to the American College of Obstetricians and Gynecologists (Dystocia and the augmentation of labor, Technical Bulletin No. 218, December 1995), neither failure to progress nor cephalopelvic disproportion are precise terms. They recommend that a more practical classification is to divide labor abnormalities into either slower-than-normal (protraction disorder) or complete cessation of progress (arrest disorder). The woman must be in the active phase of labor (cervix dilated 3 to 4 cm or more) to diagnose either of these disorders. The current criteria recommended by the American College of Obstetricians and Gynecologists (1995) for diagnosis of protraction and arrest disorders are shown in Table 14-5.

TABLE 14-5. Criteria for Diagnosis of Abnormal Labor due to Arrest or Protraction Disorders

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The American College of Obstetricians and Gynecologists (1995) has also suggested that, before the diagnosis of arrest during first-stage labor is made, the following criteria should be met: (1) the latent phase has been completed, with the cervix dilated 4 cm or more; and (2) a uterine contraction pattern of 200 Montevideo units or more in a 10-minute period has been present for 2 hours without cervical change. Calculation of Montevideo units is shown in Figure 14-3.

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FIGURE 14-3 Montevideo units are calculated by subtracting the baseline uterine pressure from the peak contraction pressure for each contraction in a 10-minute window and adding the pressures generated by each contraction. In the example shown, there were five contractions, producing pressure changes of 52, 50, 47, 44, and 49 Hg, respectively. The sum of these five contractions is 242 Montevideo units. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

Second-Stage Disorders

With achievement of full cervical dilatation, the great majority of women cannot resist the urge to “bear down” or “push” each time the uterus contracts. Typically, the laboring woman inhales deeply, closes her glottis, and contracts her abdominal musculature repetitively with vigor to generate increased intra-abdominal pressure throughout the contractions. The combined force created by contractions of the uterus and abdominal musculature propels the fetus downward. Coaching women to push forcefully, compared with letting them follow their own urge to bear down, has been reported to offer no advantage.

Causes of Inadequate Expulsive Forces

At times, the magnitude of the force created by contractions of the abdominal musculature is compromised sufficiently to prevent spontaneous vaginal delivery. Heavy sedation or epidural analgesia is likely to reduce the reflex urge to push.

MATERNAL–FETAL EFFECTS OF DYSTOCIA

Infection is a serious danger to the mother and fetus when there is prolonged labor, especially in the setting of ruptured membranes. Bacteria ascend into the amnionic fluid and invade the decidua and chorionic vessels, thus giving rise to maternal and fetal bacteremia and sepsis. Maternal fever in labor is usually due to this cause and is termed chorioamnionitis (see Chapter 15).

Abnormal thinning of the lower uterine segment creates a serious danger during prolonged labor, particularly in women of high parity and in those with prior cesarean deliveries. When the disproportion between fetal head and pelvis is so pronounced that there is no engagement and descent, the lower uterine segment becomes increasingly stretched, and uterine rupture may follow. A pathological retraction ring may also develop and may be felt as a transverse or oblique ridge extending across the uterus somewhere between the symphysis and the umbilicus.

Pelvic Floor Injury

A long-held belief is that injury to the pelvic floor muscles or their nerve supply or the interconnecting fascia is an inevitable consequence of vaginal delivery, particularly if the delivery is difficult. During childbirth, the pelvic floor is exposed to direct compression from the fetal head as well as downward pressure from maternal expulsive efforts. These forces stretch and distend the pelvic floor resulting in functional and anatomical alterations in the muscles, nerves, and connective tissues. There is accumulating concern that such effects on the pelvic floor during childbirth lead to urinary and anal incontinence, and pelvic organ prolapse later in a woman’s life.

LABOR MANAGEMENT PROTOCOLS

The National Maternity Hospital in Dublin pioneered the concept that a disciplined, codified labor management protocol reduced cesarean deliveries for dystocia. The approach is now referred to as active management of labor. Its components, or at least two of them—amniotomy and oxytocin—have been widely used, especially in English-speaking countries outside the United States.

Active Management of Labor

This term describes a codified approach to labor diagnosis and management only in nulliparous women. Labor is diagnosed when painful contractions are accompanied by complete cervical effacement, bloody “show,” or ruptured membranes. Women with such findings are committed to delivery within 12 hours. Pelvic examination is performed each hour for the next 3 hours, and thereafter at 2-hour intervals. Progress is assessed for the first time 1 hour after admission. When dilatation has not increased by at least 1 cm, amniotomy is performed. Progress is again assessed at 2 hours, and high-dose oxytocin infusion is started unless significant progress of 1 cm/h is documented.

Parkland Hospital Labor Management Protocol

During the 1980s, the obstetrical volume at Parkland Hospital doubled to approximately 15,000 births per year. In response, a second delivery unit designed for women with uncomplicated term pregnancies was developed. This provided a unique opportunity to implement and evaluate a standardized protocol for labor management. Its design was based upon the labor management approach that had evolved at the hospital up to that time, and which emphasized the implementation of specific, sequential interventions when abnormal labor was suspected. This approach is currently used in both complicated and uncomplicated pregnancies.

Women at term are admitted when active labor—defined as cervical dilatation of 3 to 4 cm or more in the presence of uterine contractions—is diagnosed or ruptured membranes are confirmed. Management guidelines summarized in Figure 14-4 stipulate that pelvic examinations be performed approximately every 2 hours. Ineffective labor is suspected when the cervix does not dilate within about 2 hours of admission. Amniotomy is then performed and labor progress evaluated at the next 2-hour examination. In women whose labors do not progress, an intrauterine pressure catheter is placed to evaluate uterine function. Hypotonic contractions and no cervical progress after an additional 2 to 3 hours result in stimulation of labor using the high-dose oxytocin regimen described in Chapter 26. Uterine activity of 200 to 250 Montevideo units is expected for 2 to 4 hours before dystocia is diagnosed.

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FIGURE 14-4 Summary of labor management protocol in use at Parkland Hospital. The total admission-to-delivery times are shorter than the potential sum of the intervention intervals because not every woman requires every intervention. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

Dilatation rates of 1 to 2 cm/h are accepted as evidence of progress after satisfactory uterine activity has been established with oxytocin. As shown in Figure 14-4, this can require up to 8 hours or more before cesarean delivery is performed for dystocia. The cumulative time required to affect this stepwise management approach permits many women the time necessary to establish effective labor and thus avoid cesarean delivery.

INDUCTION OF LABOR

Induction of labor implies stimulation of contractions before the spontaneous onset of labor, with or without ruptured membranes. Common indications for labor induction include membrane rupture without spontaneous onset of labor, maternal hypertension, nonreassuring fetal status, and postterm gestation. Elective induction of labor at term in women with a history of rapid labor or who reside an appreciable distance from the obstetrical facility is occasionally indicated. Elective induction for either convenience of the practitioner or the patient is not recommended by us, or by the American College of Obstetricians and Gynecologists. Augmentation refers to stimulation of spontaneous contractions (already occurring) that are considered inadequate because of failure of cervical dilatation or descent of the fetus.

A number of uterine, fetal, or maternal conditions present contraindications to labor induction. Most of these are similar to those that would preclude spontaneous labor and delivery. Uterine contraindicationsprimarily relate to a prior disruption such as a classical incision or uterine surgery. Placenta previa would also preclude labor. Fetal contraindications include appreciable macrosomia, some fetal anomalies such as hydrocephalus, malpresentations, or nonreassuring fetal status. Maternal contraindications are related to maternal size, pelvic anatomy, and selected medical conditions such as active genital herpes.

The condition or “favorability” of the cervix is important to the success of labor induction. Physical characteristics of the cervix and lower uterine segment as well as the level of the presenting part (station) are also important. One quantifiable method that is predictive of successful labor induction is the Bishop score (Table 14-6). Induction of labor is usually successful with a score of 9 or greater and is less successful with lower scores. Unfortunately, women frequently have an indication for labor induction with an unripe cervix. Considerable research has been directed toward various techniques to “ripen” the cervix prior to induction, and both pharmacological and mechanical methods are available for use.

TABLE 14-6. Bishop Scoring System Used for Assessment of Inducibility

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PREINDUCTION CERVICAL RIPENING

Prostaglandin E2

Local application of prostaglandin E2 gel (dinoprostone) is widely used for cervical ripening. Changes in cervical connective tissue similar to what is seen in early labor at term occur with application of prostaglandin E2 gel and include dissolution of collagen bundles and an increase in submucosal water content. Use of low-dose prostaglandin E2 increases the chance of successful labor induction, decreases the incidence of prolonged labor, and reduces oxytocin dosage.

In 1992, the Food and Drug Administration approved prostaglandin E2 gel (Prepidil) for cervical ripening in women at or near term who have an indication for induction. The gel is available in a 2.5-mL syringe that contains 0.5 mg of dinoprostone. The intracervical route offers the advantages of prompting less uterine activity and greater efficacy in women with an unripe cervix. A 10-mg dinoprostone vaginal insert (Cervidil) also was approved in 1995 for cervical ripening. The insert provides slower release of medication (0.3 mg/h) than the gel.

Administration

It is recommended that these preparations be administered either at or near the delivery suite, where continuous uterine activity and fetal heart rate monitoring can be performed. An observation period ranging from 30 minutes to 2 hours may be prudent. If there is no change in uterine activity or fetal heart rate after this period, the patient may be transferred or discharged. When contractions occur, they are usually apparent in the first hour and show peak activity in the first 4 hours. If regular contractions persist, fetal heart rate monitoring should be continued and vital signs recorded.

A minimum safe time interval between prostaglandin E2 administration and the initiation of oxytocin has not been established. According to manufacturer guidelines, oxytocin induction should be delayed for 6 to 12 hours.

Side Effects

Reported rates of uterine hyperstimulation—defined as 6 or more contractions in 10 minutes for a total of 20 minutes—are 1 percent for intracervical gel (0.5-mg dose) and 5 percent for intravaginal gel (2- to 5-mg dose). Because serious hyperstimulation or further fetal compromise can occur when prostaglandin is used with preexisting labor, such use is not generally accepted. When hyperstimulation occurs, it usually begins within 1 hour after the gel or insert is applied. Irrigation of the cervix and vagina to remove the cervical gel has not been found to be helpful. One potential advantage of the intravaginal gel is that removing the insert by pulling will usually reverse this effect. Systemic effects including fever, vomiting, and diarrhea from prostaglandin E2 are negligible.

Misoprostol

Misoprostol (Cytotec) is synthetic prostaglandin E1, and is currently available as a 100-μg tablet for prevention of peptic ulcers. It has been used “off label” for preinduction cervical ripening and labor induction. Misoprostol is inexpensive, stable at room temperature, and easily administered orally or by being placed into the vagina, but not the cervix.

Vaginal Misoprostol

Misoprostol tablets placed into the vagina are equivalent and possibly superior in efficacy to prostaglandin E2 gel. A 25-μg intravaginal dose is recommended. Uterine hyperstimulation with fetal heart rate changes is of some concern with the use of this drug. Higher intravaginal doses (50 μg or more) of misoprostol are associated with significantly increased uterine tachysystole, meconium passage and aspiration, and cesarean delivery due to uterine hyperstimulation. Reports of uterine rupture in women with prior uterine surgery preclude the use of misoprostol in these women.

Oral Misoprostol

The efficacy of oral misoprostol, 100 μg, is similar to 25 μg administered intravaginally.

MECHANICAL DILATATION OF THE CERVIX

Initiation of cervical dilatation with hygroscopic osmotic cervical dilators has long been accepted as efficacious prior to pregnancy termination. Some clinicians also use such dilators late in pregnancy to improve labor induction when the cervix is unripe.

Membrane Stripping

Induction of labor by membrane “stripping” or “sweeping” is a relatively common practice. It is considered safe and beneficial. Women who undergo membrane stripping are more likely to begin labor before 41 weeks, and fewer will require induction.

Transcervical Catheter

Transcervical balloon-tipped catheters with or without saline infusions have been used to affect cervical ripening by placement through the cervical os into the lower uterine segment. Success is varied and similar to other ripening techniques with a low risk of complication.

LABOR INDUCTION AND AUGMENTATION WITH OXYTOCIN

Synthetic oxytocin is one of the most commonly used medications in the United States. Virtually every parturient receives oxytocin following delivery, and many also receive it to induce or augment labor. The use of oxytocin given by intravenous infusion is appropriate only after assessment to exclude fetopelvic disproportion. With oxytocin induction or augmentation, it is mandatory that the fetal heart rate and contraction pattern be observed closely.

Oxytocin is avoided generally in cases of abnormal fetal presentations and of marked uterine overdistention, such as pathological hydramnios, an excessively large fetus, or multiple fetuses. Women of high parity (six or more) and women with a previous uterine scar and a live fetus are not generally administered oxytocin infusions at Parkland Hospital. The fetal condition must be reassuring, as determined by heart rate and lack of thick meconium in amnionic fluid. A dead fetus is not a contraindication to oxytocin usage unless there is overt fetopelvic disproportion.

Technique for Intravenous Oxytocin

A variety of methods for stimulation of uterine contractions with oxytocin have been employed. The woman should have direct nursing supervision while oxytocin is being infused. The goal is to affect uterine activity that is sufficient to produce cervical change and fetal descent while avoiding uterine hyperstimulation or development of a nonreassuring fetal status, or both. Contractions must be evaluated continually and oxytocin discontinued if they exceed five in a 10-minute period or seven in a 15-minute period; if they last longer than 60 to 90 seconds; or if the fetal heart rate pattern becomes nonreassuring. With hyperstimulation, immediate discontinuation of oxytocin nearly always rapidly decreases the frequency of contractions. When oxytocin is stopped, its concentration in plasma rapidly falls because the mean half-life is approximately 5 minutes.

Synthetic oxytocin is usually diluted into 1000 mL of a balanced salt solution that is administered by infusion pump. Administration by any other route is not recommended for labor stimulation. To avoid bolus administration, the infusion should be inserted into the main intravenous line close to the venipuncture site. A typical oxytocin infusion consists of 10 to 20 units—equivalent to 10,000 to 20,000 mU—mixed into 1000 mL of lactated Ringer solution, resulting in an oxytocin concentration of 10 or 20 mU/mL, respectively.

Oxytocin Dosage

As shown in Table 14-7, there are a number of oxytocin regimens considered appropriate for labor stimulation.

TABLE 14-7. Oxytocin Regimens for Stimulation of Labor

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At Parkland Hospital, oxytocin is initiated at 6 mU/min and increased at 40-minute intervals to 42 mU/min as needed. When uterine hyperstimulation occurs, the infusion rate is halved.

Montevideo Units

The American College of Obstetricians and Gynecologists recommends that prior to diagnosing an arrest of first-stage labor, the uterine contraction pattern should exceed 200 Montevideo units for 2 hours without cervical change (Figure 14-3). Some experts have suggested that 4 hours is a more appropriate interval in otherwise uncomplicated labors. More data are needed regarding the precise safety and efficacy of such contraction patterns in women with a prior cesarean delivery, with twins, with an overdistended uterus, and in those with chorioamnionitis.

AMNIOTOMY

Amniotomy or artificial rupture of the membranes, also referred to in Britain as surgical induction, is commonly used to induce or augment labor. Other common indications for amniotomy include internal electronic fetal heart rate monitoring when fetal jeopardy is anticipated and intrauterine assessment of contractions when labor has been unsatisfactory. Elective amniotomy to hasten spontaneous labor or detect meconium is also acceptable and commonly practiced.

Several precautions to minimize the risk of cord prolapse should be observed when membranes are ruptured artificially. Care should be taken to avoid dislodging the fetal head. Having an assistant apply fundal and suprapubic pressure may reduce the risk of cord prolapse. Some prefer to rupture membranes during a contraction. The fetal heart rate should be assessed prior to and immediately after the procedure.


For further reading in Williams Obstetrics, 23rd ed.,

see Chapters 17, “Normal Labor and Delivery,” 20, “Dystocia—Abnormal Labor,” and 22, “Induction of Labor.”