Abnormal Labor

Etiology

Normal labor progression requires consideration of the 3 "Ps," representing power, passage, and passenger. Power refers to uterine contractions and maternal expulsive efforts. The passage is the maternal pelvis, and the fetus is the passenger. Abnormal labor progression may be related to one or more of these factors. The size, position, and presentation of the fetus and the adequacy of the maternal pelvis are tested as uterine contractions provide propulsion. Asynclitism or extension of the fetal head, occiput posterior or transverse positions, and mentum or brow presentations may contribute to or be sole etiologies of abnormal labor progression or labor dystocia.[4] Labor abnormalities due to unfavorable fetal or maternal pelvic dynamics may require a cesarean delivery.[5]

Uncommonly, a Bandl ring may be associated with protracted or arrested labor. A Bandl ring is a constriction between the thicker upper contractile and thinner lower uterine segments. It is unclear why this constriction develops during parturition, although prolonged labor and labor dystocia have been suggested as possible causes.[6]

Risk Factors Associated with Abnormal Labor

Maternal Factors

• Advanced maternal age
• Bandl ring
• Chorioamnionitis
• Epidural analgesia
• Gestational diabetes
• Hypertensive disorders
• Inadequate uterine contractions
• Lack of supportive care during labor
• Large weight gain in pregnancy
• Non-gynecoid maternal pelvimetry
• Nulliparity
• Poor hydration and nutrition during labor
• Post-term pregnancy
• Short stature

 Fetal Factors

• High fetal station at full cervical dilation
• Macrosomia
• Malpresentation
• Nonreassuring fetal heart rate tracing[7]

Epidemiology

Pelvic shape varies significantly among women, and the fit between the maternal pelvis and fetus is exceptionally tight, unlike in other primates.[8] Approximately 20% of all labor involves protraction and/or arrest disorders. Disorders of labor are the most common reason for primary cesarean sections.[9]  

Abnormal labor involving true labor dystocia can result in fetal and/or maternal injury and even death. The trend toward more cesarean sections in developed countries has drawn attention to initiatives to minimize primary cesarean sections safely. In this regard, guidelines have been developed to implement more conservative management strategies for the first and second stages of labor.[10]

After the delivery of the fetus, the third stage of labor culminates in the delivery of the placenta. Abnormalities in the third stage can lead to postpartum hemorrhage requiring blood transfusions and, at times, resulting in maternal morbidity and mortality. Postpartum hemorrhage affects approximately 5% of all deliveries. The mean duration of the third stage of labor is 5 to 6 minutes when utilizing active management, and 90% of the third stages take less than 10 minutes. Postpartum hemorrhage is more likely when the third stage lasts more than 18 minutes and significantly more likely when the third stage lasts over 30 minutes.[11] 

Pathophysiology

Friedman initially established a labor curve for the first stage of labor; however, it has since been contemporized by Zhang et al.[12] Recent reference graphs indicate that the latent phase of labor is much longer than initially thought, and the active stage of labor begins at 6 cm of cervical dilation. Induced labor tends to have a longer latent phase in the first stage of labor when compared to spontaneous labor. However, the active phase of the first and second stages do not vary significantly, respective of spontaneous or induced labor. Insufficient uterine activity is the most common reason for a labor abnormality in the first stage of labor.[13] 

Labor is a physically intensive process, and prenatal exercise may play a role in increased uterine contractility. Increased levels of physical activity have been associated with higher levels of oxytocin and decreased needs for induction and augmentation of labor.[14] Maternal inactivity during pregnancy may be associated with prolonged labor. In one particular prospective cohort study, patients who were physically active during pregnancy were less likely to have a prolonged latent phase of labor and more likely to have a shorter active phase of labor.[14] For these and other reasons, regular prenatal exercise is highly encouraged.

The pathophysiology of abnormal labor is otherwise poorly understood. There is likely a multitude of pathophysiologic etiologies. Current management and intervention are limited and, at times, unsuccessful. Treatment options that target the specific and multiple underlying etiologies may be more successful in treating labor abnormalities. Additional knowledge of diagnostic tools and individualized therapies may lead to future new clinical opportunities.[15] 

Toxicokinetics

Oxytocin is a key pharmacologic agent in the augmentation of inadequate uterine contractions and, thus, in treating abnormal labor. It is one of the most commonly used medications in obstetrics. Exogenous oxytocin responsiveness typically commences at 20 weeks of pregnancy and increases with advancing gestational age until 34 weeks, when it appears to plateau.[16] Oxytocin has a short plasma half-life of 3 to 6 minutes. Oxytocin is utilized during labor as a continuous intravenous infusion.

Adverse maternal effects associated with oxytocin use in labor include flushing, nausea, vomiting, headache, tachycardia, and hypotension.[17] Maternal water retention and hyponatremia rarely occur due to oxytocin use and only in extended exposures to a high oxytocin dose, mainly if administered in hypotonic solutions. This hyponatremia is due to oxytocin's structure being similar to vasopressin and causing cross-reactions with renal vasopressin receptors.[18] 

Uterine tachysystole commonly occurs as a result of oxytocin administration during labor. Tachysystole is 6 or more contractions in a 10-minute window, averaged over 30 minutes. Without adequate time for uterine relaxation between contractions, there is a potential for decreased fetal oxygenation.[17] Due to the high risk of patient harm when used in error, oxytocin is a high-alert medication.[19]

History and Physical

Labor begins with regular uterine contractions, resulting in cervical effacement and dilation. An essential piece of the patient history to obtain is the start time of contractions and their occurrence interval. An abdominal examination is a critical component of an obstetric exam as it provides an estimated fetal weight and an assessment of fetal presentation. The continuous monitoring of uterine activity discerns how frequent contractions are occurring. Internal pressure catheter monitoring measures the actual strength of uterine contractions. Fetal heart rate tracing provides reassurance of fetal well-being throughout the labor process.

Digital vaginal exams, conducted at different intervals throughout labor, assess maternal pelvimetry, bony pelvis shape, and capacity, as well as cervical effacement and dilation. Serial digital exams are used to assess the fetal position, station, and descent of the presenting part, which is crucial in determining the normal or abnormal progression of labor.[20]

Evaluation

Determining the progression of labor is a critical component of intrapartum care. Maternal uterine activity is assessed by manual palpation, external tocodynamometry, or monitoring with an intrauterine pressure catheter. Using external tocodynamometry, target uterine activity is 3 to 5 contractions in a 10-minute window. Effective contractions last 30 to 40 seconds. Intrauterine pressure assessment using a catheter is commonly used to measure the strength or intensity of uterine contractions in patients with ruptured membranes. This technique involves inserting a catheter through the cervix and into the uterus, which allows for direct measurement of intrauterine pressure. This monitoring employs Montevideo units (MVUs) as measurements of contraction intensity. Montevideo units are calculated by adding the sum of the net contraction pressures over 10 minutes. Adequate uterine activity is targeted at 200-250 MVUs.[21] Although this method of assessing uterine contractions has limitations, a more useful and accurate system has not yet been devised. Electrohysterography is a recent technique using externally applied abdominal electrodes, which monitors and records electrical voltages generated by myometrial contractions. This method of assessing uterine contractility is not currently part of standard clinical practice.[21]

Evaluating the fetal position is another crucial component of intrapartum care and management. Several studies have shown increased adverse outcomes when the fetus is in an occiput posterior position, including a more prolonged second stage of labor and increased cesarean delivery rate. Randomized trials have shown that manual rotation of the fetus from an occiput posterior position to an occiput anterior position can reduce the length of the second stage of labor.[22] Manual rotation is more effective than maternal repositioning in labor, and it involves less technical skill and less risk than instrumental rotation.[22]

Treatment / Management

Abnormal labor progression is associated with multiple poor maternal and fetal outcomes. Thus, active management is imperative to avoid adverse outcomes for the mother and baby. Most labor and delivery units have an established protocol for administering oxytocin to augment inadequate contractions, which entails the administration of the proper medication and dosage and criteria for an incremental increase as clinically warranted. These protocols also include monitoring maternal and fetal status for adverse events. With a protracted or arrested second stage of labor, operative vaginal delivery with either forceps or vacuum by a skilled obstetric professional may be an option to expedite delivery. Cesarean delivery may be necessary in other labor prolongation, protraction, and arrest cases, especially in the active phase of labor's first stage. For a prolonged third stage of labor, manual placenta extraction is sometimes required. 

Prolonged or Protracted First Stage

Latent Phase

• This phase can be prolonged for many hours and even days. Hence, the decision to admit the patient to the hospital depends on various factors, including the status of the cervix, the emotional state of the patient, associated complications, tolerance to pain, and the patient's distance from the hospital. Increased obstetrical interventions have been associated with admission during the latent phase of labor. Therefore, it is imperative to consider the options of admission versus continued outpatient management of the latent phase of labor.[23] 
• Therapeutic rest may be considered if desired. Morphine may be administered at 5 to 10 mg intramuscularly and intravenously simultaneously, with a maximum total dose of 20 mg. Alternatively, intramuscular morphine sulfate and promethazine may be utilized.[24]
• Oxytocin and amniotomy may be helpful in actively treating prolonged latent phase.

Active Phase

• For women in the active phase of the first stage, oxytocin may be administered, and an amniotomy may be performed.
• If there has been no cervical change after 4 hours of adequate (>200 Montevideo units) uterine contractions and ruptured membranes or 6 hours with adequate uterine contractions, ruptured membranes, and oxytocin augmentation, it is advisable to proceed with cesarean delivery.
• However, if labor progresses slowly or normally, oxytocin administration is continued.

Protracted or Arrested Second Stage

• Oxytocin augmentation is started for minimal (<1 cm) or absent descent after 60 to 90 minutes of maternal pushing with less frequent uterine contractions.
• Without epidural anesthesia, nulliparous women can push for at least 3 hours and multiparous women for at least 2 hours before considering operative intervention. As long as the fetus continues to descend and/or rotate to a favorable position for spontaneous vaginal delivery and the fetal heart rate pattern is reassuring, any operative intervention should be delayed.
• Manual rotation of the fetus from occiput posterior presentation to occiput anterior presentation can be attempted if needed in a protracted second stage.
• An additional hour of pushing may be allowed in women with epidural anesthesia before considering operative intervention.
• Operative vaginal delivery may be utilized when the second stage of labor is protracted.[25]

Differential Diagnosis

• Abdominal Pain
• Bandl ring
• Braxton Hicks contractions
• Cervical stenosis
• Chorioamnionitis
• Placental abruption
• Premature rupture of membranes
• Prodromal labor
• Uterine Mullerian anomaly
• Uterine rupture

Prognosis

The prognosis of abnormal labor varies according to the stage. With a prolonged first-stage latent phase, intervention with amniotomy and oxytocin can be helpful and allow normal progression for the remainder of labor. These same strategies may be utilized with abnormalities in the active phase of the first stage of labor. A recent randomized controlled trial of early amniotomy, within 1 hour of cervical balloon catheter expulsion, shows over 2 times faster labor than waiting longer to perform amniotomy in term patients induced with mechanical cervical ripening.[26]

Maternal and neonatal morbidity is associated with a prolonged first stage of labor, including maternal fever from endometritis, shoulder dystocia, hemorrhage, and blood transfusion. [27][28] With an abnormal second stage of labor, oxytocin may be utilized again. An increase in third- and fourth-degree perineal lacerations, cesarean delivery, and a prolonged hospital stay is associated with a protracted or arrested second stage of labor.[29][28] The fetus has an increased risk of NICU admission, low Apgar scores, and a need for assisted ventilation with abnormal labor. [29]

Operative vaginal delivery may expedite the abnormal labor process in certain situations. Based on recommended criteria, cesarean delivery is indicated with abnormal labor in the first and second stages.[30] Current methods to prevent injury, blood loss, and infection result in a good prognosis when cesarean delivery is necessitated.[22]

Complications

Roughly 20% of labors are associated with prolongation, protraction, or an arrest abnormality. Various adverse maternal and fetal outcomes are associated with labor abnormalities. Infection, operative birth, low Apgar score at 5 minutes, prolonged hospitalization for both mom and baby, third- and fourth-degree maternal perineal tears, above-average blood loss, and NICU (neonatal intensive care unit) admission are all increased with abnormal labor progress.[31] 

With abnormal labor being the main indication for cesarean delivery during labor, the perceived safety of the cesarean delivery must be addressed. With the increasing rate of cesarean deliveries in the United States, there has been a concomitant increase in abnormal placentation, specifically placenta accreta and placenta previa, as well as cesarean scar pregnancies. Along with these complications come a paralleled increase in maternal morbidity and mortality, as placenta accreta and placenta previa account for a large percentage of maternal morbidity and mortality.[32]