eMedicine Specialties > Obstetrics and Gynecology > Labor and Delivery

Abnormal Labor

Author: Saju Joy, MD, MS, Assistant Professor, Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Wake Forest University School of Medicine
Coauthor(s): Patricia L Scott, MD, Fellow in Maternal-Fetal Medicine, Wake Forest University, Bowman Gray School of Medicine; Deborah Lyon, MD, Director, Division of Gynecology, Associate Professor, Department of Obstetrics and Gynecology, University of Florida Health Science Center at Jacksonville
Contributor Information and Disclosures

Updated: Dec 19, 2008

Introduction

Background

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 3 stages of labor.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.
• The second stage of labor is defined as complete dilation of the cervix to the delivery of the infant.
• The third stage of labor involves delivery of the placenta.

See Media files 1-2 for the normal labor curves of both nulliparas and multiparas. The following table shows abnormal labor indicators.

Open table in new window

First stage of labor

Latent phase: Definitions for prolonged latent phase are outlined in the table above. Diagnosis of abnormal labor during the latent phase is uncommon and likely an incorrect diagnosis.

Active phase: 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. Abnormalities of cervical dilation (protracted dilation and arrest of dilation) as well as descent abnormalities (protracted descent and arrest of descent) are outlined in the table above. 

In general, abnormal labor is the result of problems with one of the 3 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)

See Causes.

Pathophysiology

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.⁶

Frequency

United States

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.

Mortality/Morbidity

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.

Clinical

History

• 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 Media files 1-2).

Physical

• 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.

Causes

• 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.
• 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 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.7 Intravenous oversedation has also been implicated as prolonging labor in both the latent and active phases.

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