INTRODUCTION	Section 2 of 9
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Background: Dystocia is defined as abnormal or difficult labor. The opposite of dystocia is eutocia, which is normal labor. Dystocia is often an indication for operative delivery, with its associated complications. Therefore, a diagnosis of dystocia has a significant impact on the health care system. Dystocia entails a vast number of influencing factors that include both maternal and fetal entities. Accurately diagnosing dystocia is crucial. The cesarean delivery rate for the past 20 years in the United States has consistently been 50-75% higher than the rate in Europe. The increase in the cesarean delivery rate in the United States is believed to be partly due to the fear of litigation, influencing physicians to perhaps prematurely diagnose dystocia and other indications. Interestingly, in some European countries, the cesarean delivery rate is rising in response to the increasing incidence of litigation in those countries.

Although simultaneous improvement in perinatal mortality and morbidity rates has been documented, this improvement has not been the result of an increased rate of cesarean deliveries. From an economic standpoint, vaginal delivery is less expensive than abdominal delivery in regard to both the delivery itself and the hospital stay. In this article, maternal and fetal factors relating to dystocia and its management are identified, discussed, and analyzed.

Another form of dystocia that has been the subject of significant discussion but not numerous scientific analyses is shoulder dystocia. No consensus has been reached on a definition for shoulder dystocia. Shoulder dystocia is said to occur when the fetal bisacromial diameter cannot negotiate the pelvic brim (the obstetric conjugate). Some may define shoulder dystocia as when the anterior shoulder cannot pass the pubic symphysis. Further, some have described shoulder dystocia as occurring when special maneuvers are applied to deliver fetal shoulders.

Pathophysiology: To characterize a labor as abnormal, a basic understanding of normal labor is necessary. Normal labor starts with regular uterine contractions sufficient to result in cervical effacement and dilatation. Early in labor, uterine contractions are irregular and cervical effacement and dilatation are gradual. The active phase of labor commences when cervical dilatation reaches 4 cm and uterine contractions are more powerful. Studies by Friedman from the 1950s were among the first stepping-stones in the understanding of normal labor. His studies established the minimum criteria for dilatation of the cervix during the active phase of labor. For nulliparous women, cervical dilatation of 1.2 cm/h is accepted as the norm. For parous women, a minimum of 1.5 cm/h of cervical dilatation is accepted as the norm. Remembering that these rates of cervical dilatation are considered an average and that the active phase of labor does not necessarily start at a specific dilatation is of the utmost importance.

Dystocia is considered the result of any of the following during labor: (1) abnormalities of expulsive forces; (2) abnormalities of presentation, position, or development of the fetus; and (3) abnormalities of the maternal bony pelvis or birth canal. Frequently, combinations of these 3 interact to produce a dysfunctional labor.

Frequency:

• In the US: In the United States, 25-30% of deliveries are cesarean, and dystocia accounts for 30% of all primary cesarean deliveries. Previous cesarean delivery is the second most common indication for cesarean delivery, followed by a nonreassuring fetal heart rate. In current practice, most patients with a prior cesarean delivery undergo repeat cesarean delivery. Two factors dictate whether vaginal birth after cesarean delivery (VBAC) is pursued. One is that the safety of VBAC is not as promising as once thought in late 1980s and early 1990s. The second is that even among the best candidates for VBAC, complications can occur that may result in litigation. Considering that the overall cesarean birth rate is 30%, with 30-40% due to dystocia, then the rate of dystocia in pregnant US women is 10%.

  With regard to shoulder dystocia, the subjectivity of the definition is the main reason for the variable prevalence reported in the literature. Overall, the prevalence of shoulder dystocia is 5 cases per 1000 deliveries.

• Internationally: Because the diagnosis of dystocia is subjective in most cases, the exact frequency with which it occurs in most parts of the world remains unknown.

Mortality/Morbidity: Dystocia is associated with increased maternal and fetal mortality and morbidity.

• Treatment for dystocia, which includes cesarean delivery, is also associated with increased maternal mortality and morbidity, including damage to other organs, impairment of future fertility, and wound infection. Increased blood loss and possible blood transfusion also add to the morbidity. Although increased blood loss may not seem to be a profound and disturbing complication of dystocia in developed countries, it certainly poses a major impact on maternal health and the health care system in developing countries. The same is true regarding surgical site infection, which is a more significant problem in developing countries.

• Infant morbidity is minimal and is mostly related to iatrogenic lacerations upon performance of the hysterotomy incision during a cesarean delivery. However, infants delivered via cesarean are at higher risk for transient tachypnea of the newborn. In some cases, shoulder dystocia has also been reported to cause neonatal brachial plexus palsy (NBPP). Fortunately most cases of NBPP are transient; however, in 10% of the cases it can be permanent. NBPP has also been reported after cesarean delivery. As a result, some investigators have concluded that labor itself may be a cause for NBPP.

Race: The prevalence of dystocia is not generally associated with any particular race. Shy and colleagues studied the relationships among maternal birth weight, race, and risk for cesarean delivery in nulliparous women. In this population-based cohort study, low and high maternal birth weights were found to exert an intergenerational risk for non-Hispanic white females only in regard to dystocia with subsequent abdominal delivery; however, remember that the criteria for a diagnosis of dystocia can vary in different parts of the world.

	CLINICAL	Section 3 of 9
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History:

• Besides a complete history, special attention is necessary to determine if a patient is in active labor. Labor is defined as the onset of regular contractions that cause cervical dilatation. The patient must be questioned about the onset of regular contractions and the frequency, intensity, and duration of the contractions.

• The patient's last cervical examination findings must be available to use for a comparison.

• Past obstetric history and prenatal events should be investigated.

Physical:

• A careful and thorough physical examination must be performed. The examination must include a complete abdominal examination with Leopold maneuvers in order to ascertain the presentation of the fetus and to estimate the fetal weight. Clinical estimation of the fetal weight may not be accurate, especially at higher ranges of birth weight. This is also the case with ultrasound estimation of fetal weight. Note that the margin of error increases with gestational age and weight, especially for fetuses heavier than 4-4.5 kg.

• The focus of the pelvic examination is broader than just determining the degree of cervical dilatation, effacement, and station. Pelvic examination provides an excellent opportunity to assess the patient's pelvis and to perform clinical pelvimetry. Finally, fetal presentation and position must be assessed. Evaluating the position once cervical dilatation is advanced or completed appears to be a common practice, but knowing the fetal position as early as possible is beneficial because fetal position can be a major contributor to dystocia.

• The examiner, preferably the same person each time, should perform a careful cervical examination every 2 hours once the patient is in active labor (certain clinical situations may require modification of this time interval). All of the above named factors (ie, cervical dilatation, effacement, station, position, presentation) should be reevaluated during each examination. A diagnosis of dystocia should be made in a prompt manner so that an attempt to correct the dysfunction can be made without jeopardizing the mother or fetus.

Causes: Normal labor is a coordinated interplay between maternal expulsive forces (power), fetal position (passenger), and maternal pelvic shape and structure (passage); therefore, before making a diagnosis of dystocia, evaluating each of these 3 parameters (Ps) is important. One or more of these factors can contribute to dystocia.

• Abnormalities of maternal expulsive forces (power)

• The first criterion for diagnosis of an abnormality of the expulsive forces is that the patient must be in the active phase of labor, which is defined as a phase of maximal cervical dilatation. With adequate contractions in the active phase, a cervical dilatation rate of at least 1.2 cm/h in nulliparous women and 1.5 cm/h in parous women can be expected. For most women in spontaneous labor, these rates of cervical dilatation are achieved with at least 3-5 contractions in a 10-minute period. If the rate of dilatation is less than expected, the diagnosis is a protraction disorder. If the evaluation has demonstrated no cervical dilatation in a 2-hour period, the diagnosis is arrest of dilatation.

• Before determining a diagnosis of arrest of dilatation, the adequacy of contractions must be evaluated. The rate of uterine contractions should be at least 3 every 10 minutes to be considered minimally effective. The intensity of contractions should also be at least 25 mm Hg above the baseline. The health care provider attending the labor can perform the assessment of these characteristics of uterine activity. An intrauterine pressure catheter can be used to measure the adequacy of the uterine contractions. Intensity is measured in Montevideo units, which are calculated as the intensity of contractions in millimeters of mercury multiplied by the frequency for a 10-minute period. An adequate contraction pattern exceeds 200 Montevideo units in a 10-minute period. If this pattern is present for 2 hours without cervical change, the diagnosis of arrest of dilatation can safely be made.

• The effect of anesthesia on the pattern of labor has been extensively reviewed. Recent studies indicate that epidural anesthesia prolongs the active phase and the second stage of labor. Despite these findings, studies have noted neither an increase in nor correlation of epidural anesthesia and the rate of cesarean delivery. However, a few studies suggest an increased prevalence of malpresentation and operative vaginal deliveries.

• Abnormalities of fetal presentation, position, and development (passenger)

• Any presentation other than occiput increases the probability of dystocia. In face or brow presentations, dystocia can develop with mentum posterior face presentations. With these presentations, flexion of the head is impeded by compression of the fetal brow under the symphysis pubis.

• As labor progresses, the examiner should ascertain if asynclitism (the relationship between the anterior and posterior parietal bones and the sagittal suture with the maternal pelvis) is present. If one of the parietal bones precedes the sagittal suture, the head is considered asynclitic. When asynclitism is persistent in either the occiput anterior or the posterior position, forceps-assisted vaginal delivery can be helpful for correcting the problem. Kielland forceps is the most commonly used type of forceps for this purpose. The sliding lock of the instrument allows accurate cephalic application followed by correction of the asynclitism; however, other types of obstetrical forceps can also be used. Persistent occiput posterior position, leading to a prolonged second stage, can also be corrected by performing a forceps-assisted vaginal delivery.

	WORKUP	Section 4 of 9
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Imaging Studies:

• Imaging studies are rarely used to evaluate dystocia. In the past, x-ray pelvimetry was routinely performed, but this practice has fallen out of favor because of the potential radiation exposure hazards to the fetus. Also, the clinical utility of such studies has not been proven in controlled trials. Other imaging modalities such as ultrasonography and MRI have been used but, again, are of little clinical utility.

• Thurnau and colleagues have suggested a fetal-pelvic index measurement using ultrasonographic and radiographic studies to help predict fetal-pelvic disproportion. Maternal pelvic diameters are measured through radiographs, and fetal head sizes are measured through ultrasonograms. The transverse and anteroposterior diameter of the pelvic inlet and mid pelvis are measured, then the inlet and midpelvic circumferences are calculated. The same protocol is used for the fetus, and head and abdominal circumferences are measured. The difference between the fetal and maternal circumferences is calculated. The sum of the 2 most positive fetal-pelvic circumference differences is calculated as the fetal-pelvic index.

• Ultrasonograms have also been used to evaluate fetal abdominal size and shoulder diameter to predict shoulder dystocia, but with very low sensitivity and specificity.