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 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 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]}

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.

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.