NORMAL PROGRESS OF LABOR
Labor can be defined as progressive dilatation of the cervix in
association with repetitive uterine contractions. This process is illustrated in
Figure 58-4
. Labor can
be either
Figure 58-4
Composite of the average dilation curve for nulliparous
labor based on analysis of the data derived from the patterns traced by a large,
nearly consecutive series of gravidas. (Redrawn with modification from Friedman
EA: Primigravid labor: A graphicostatistical analysis. Obstet Gynecol 6:1567,
1955.)
spontaneous or induced, and the activity of the uterus is measured in frequency,
duration, and intensity of contractions. Although labor is a continuous process,
it has traditionally been divided into three stages. The first stage of labor consists
of at least two phases: a latent phase of variable duration (which is defined as
the period between the onset of labor and the point at which a change in the slope
of cervical dilatation is noted) and a phase of maximal dilatation (which usually
begins at approximately 3-cm dilation).[57]
During
the active phase of labor, uterine contractions occur approximately every 3 minutes,
have a duration of about 1 minute, and achieve an intrauterine pressure of 50 to
70 mm Hg. The most commonly used measure of uterine activity by obstetricians, however,
is the Montevideo unit, which is defined as the average intensity frequency per 10
minutes. During the normal progress of labor, the cervix should dilate at a rate
of approximately 1 cm/hr. When uterine activity is not optimal (contractions less
than 50 mm Hg every 3 minutes or below 250 Montevideo units), an oxytocic drug is
usually administered by the obstetrician. The second stage of labor is defined as
the interval between full cervical dilation and delivery of the infant. The duration
of this second stage is typically 1 to 2 hours, with allowances made for epidural
analgesia. The third stage of labor encompasses delivery of the placenta. The progress
of labor may be abnormal and can be classified as a slow latent phase, arrest of
active phase, and arrest of descent. Table
58-5
lists the diagnostic features of abnormal progress of labor.
The impact of neuraxial analgesia techniques on uterine activity
and the progress of labor is a controversial issue. An association between epidural
analgesia and cesarean delivery is based on uncontrolled and confounding variables.
Actually, women with abnormal labor may have more than expected pain early in labor;
therefore, they may require epidural analgesia for what is already
TABLE 58-5 -- Diagnostic features of prolonged labor
|
Feature |
Nulliparas |
Multiparas |
|
Slow latent phase |
>20 hr |
>14 hr |
|
Slow-slope active phase |
>1.2 cm/hr |
<1.5 cm/hr |
|
Active phase arrest |
No cervical dilation for 2 hr |
|
|
Slow descent of fetus |
<1 cm/hr |
<2 cm/hr |
|
Arrest of descent |
No descent for 1 hr |
|
|
Second Stage |
|
|
Nulliparous |
More than 2 hr without regional analgesia |
|
|
More than 3 hr with regional analgesia |
|
|
Multiparous |
More than 1 hr without regional analgesia |
|
|
More than 2 hr with regional analgesia |
|
|
From Glosten B: Anesthesia for Obstetrics. In
Miller RD (ed): Anesthesia, 5th ed. New York, Churchill Livingstone, 2000, p 2033. |
a dysfunctional labor with a more frequent risk of cesarean section. Prospective
randomized trials have been undertaken to evaluate the controversy between epidural
and cesarean deliveries; whereas earlier studies did show an association, more recent
studies have demonstrated no significant differences between groups in the rate of
cesarean delivery.[58]
Furthermore, though methodologically
limited, several studies of sentinel events that evaluated the impact of the introduction
of an epidural service on a stable labor and delivery unit have failed to demonstrate
a change in the cesarean section rate.[59]
A recently
published meta-analysis of these sentinel event studies that included more than 37,000
parturients showed that the sudden availability and high utilization of an epidural
service had no effect on the incidence of cesarean delivery or cesarean deliveries
performed for dystocia.[60]
