Normal and Abnormal Labor

Normal and Abnormal Labor

Anat Shmueli


All approaches to the systematic assessment of labor are grounded in the concept that it is possible to evaluate the progress of labor by analyzing the relationships among cervical dilation, fetal descent, and elapsed hours in labor.1,2,3,4,5,6,7 This kind of graphic analysis forms the basis for clinical decision-making in dysfunctional labor, but it must be used in conjunction with other obstetric data. Thus, information about pelvic architecture, uterine contractility, fetal size, position, and attitude, and the state of fetal oxygenation must be integrated with labor curve data if the obstetrician is to make the most informed and appropriate clinical judgments.

To a large extent, intrapartum decision-making is a process of estimating seriatim the probability of a safe vaginal delivery as updated information is obtained during the course of labor. All of the clinical data used in reaching decisions about obstetric interventions (especially the use of oxytocin, conduction anesthesia, cesarean delivery, and instrumental delivery) should be viewed from this perspective.

Normal Cervical Dilation

The relationship between cervical dilation and the time elapsed in labor is traditionally described by a sigmoid-shaped curve, the “Friedman curve”.1,2,3 Dilation is divided into a latent phase and an active phase. The latent phase extends from the onset of labor until the upward inflection in the curve and is associated with little incremental change in dilation. According to the Friedman curve, enhancement in the rate of dilation begins at the onset of the active phase, during which most cervical dilation occurs. A gradual increase in dilation (the acceleration phase) initiates the active phase and leads, usually in about an hour, to a period of more rapid and linear dilation (the phase of maximum slope). During the terminal part of the active phase, the deceleration phase, dilation appears to slow. Contemporary labor curves show no deceleration phase, and allow a slower cervical dilation throughout the active phase (Figure 44.1).8 In fact, the cervix continues to open at a constant rate, but as it retracts around the head (which has begun to descend) to achieve complete dilation, its movement is directed cephalolaterally. This change cannot be appreciated readily by examination with the fingers because only radial changes in distance can be determined.

By convention, full cervical dilation is considered to be 10 cm. This is a clinically useful approximation; however, because the cervix does not generally dilate to more than the largest diameter of the object passing through it, full dilation for most term babies is somewhat less, whereas for exceptionally large babies it may be more. This issue is of particular importance when interpreting the labor curves of very premature fetuses whose head diameter may be considerably less than 10 cm; in these cases, the curve of dilation is necessarily foreshortened.

In general, the labors of multiparas are shorter than those of nulliparas, with similar progression pace during the latent phase of labor and more rapid rates of dilation and descent in the active phase and second stage.9 Criteria for all multiparas are the same, irrespective of how many babies they have had.1,2,3 Multiparas who have had all previous babies by cesarean delivery should be judged by nulliparous criteria during labor.10

Latent Phase

During the latent phase, the cervix is prepared for the more rapid dilation that will occur later.3,11,12 Physical changes take place that can be appreciated clinically; these constitute what has been referred to as ripening or maturation of the cervix and, in some patients (particularly nulliparas), occur largely or completely before the onset of labor. The duration of the latent
phase is inversely proportional to the degree of prelabor cervical maturation. The palpable softening, effacement, and anterior rotation of the cervix in the pelvic axis that occur during the latent phase are prerequisites for entering active phase dilation.

Measurement of the duration of the latent phase requires knowledge of the time of onset of labor, which cannot always be determined with certainty. It is reasonable to use the time at which the patient began to perceive regular uterine contractions as an approximation. According to contemporary labor curves published in 2010, the 95th percentile for the length of the latent phase of labor is similar in multiparas and in nulliparas. Nonetheless, a larger cervical dilation at the beginning of the latent phase usually results in a shorter latent phase.9 For the same reason, the latent phase is often shorter in multiple gestations, hydramnios, or after removal of a cervical cerclage.

During the latent phase, the cervix dilates slowly, or not at all. Although previously believed that the shift to active phase often occurs at approximately 3 to 5 cm of cervical dilation,1,13 a newer labor pattern, based on a large contemporary cohort, demonstrates that the transition to active phase occurs at 6 cm of dilation in both nulliparous and multiparous women9 (Figure 44.2).

Active Phase

Cervical dilation during the active phase is much more rapid than in the latent phase and tends to accelerate as labor advances. To determine normality, clinical assessment of labor requires measuring the speed at which the cervix dilates during the active phase. When two observations of cervical dilation have been made during this period of linear change, the slope of the dilation line can be calculated.

Abnormalities of active phase are defined by deviations from this projected rate of dilation. These deviations in cervical dilation in the active phase can be readily identified if serial observations of cervical dilation and fetal station are plotted on square-ruled graph paper. Observation and calculation of slopes are relatively simple.1 Paradigms exist in order to use the graphic system without the need for calculations4,5,6; some electronic medical records automate these calculations.

Full cervical dilation occurs when the cervix retracts to the widest diameter of the presenting part. Usually, the cervix retracts symmetrically, but sometimes a segment lingers, particularly anteriorly, in the presence of deflexed attitudes of the head. Full dilation should not be diagnosed until
the entire cervix has retracted spontaneously to or beyond the widest diameter of the leading part of the fetus.

There is a linear relationship between the presenting part’s station and cervical dilation, with a general trend of descent of the fetal presenting part with increasing dilation during the first stage of labor.14

Normal Fetal Descent

As the cervix dilates in late active phase, resistance to fetal descent decreases and the force of uterine contractions, coupled after complete dilation with active maternal bearing-down efforts, begins the expulsion of the fetus. The efficiency and normality of the descent mechanism can be judged from the rate of descent. If the relationship between fetal descent and elapsed time in labor is plotted graphically, it is apparent that descent also has a latent phase during which little in the way of descent occurs under most circumstances (Figure 44.1). The degree of descent that has occurred before the onset of labor has prognostic importance for the probability of vaginal delivery. In fact, an unengaged head at the onset of labor is associated with a considerably increased risk for cesarean delivery.15 Lately, several sonographic markers for evaluation of fetal station before and during labor have been suggested, including head-perineum distance, angle of progression, and head-symphysis distance.16

Considerable fetal descent may sometimes occur during the latent phase. Multiparas often commence labor with the presenting part at a relatively high station, and appreciable descent takes place in the latent phase. Of utmost importance in this regard is that lack of fetal descent before active phase labor is not an evidence of a labor aberration or of fetopelvic disproportion.

Uterine Activity in Labor

Initially, contractions are often mild and somewhat irregular; they become progressively more intense, frequent, and regular as the latent phase progresses. However, this is not always the case and a broad range of contraction patterns may be observed in the normal latent phase, including very intense and frequent contractions. It is usually impossible to identify the transition from latent phase to active phase labor solely on the basis of uterine activity.

During the active phase, contractions are generally more frequent and of greater amplitude and duration than in the latent phase. However, a large spectrum of contractile patterns exists during normal labor,17 and there is no reliable or predictive means to identify dysfunctional dilation or descent by observing uterine activity. Consequently, the clinical identification of dysfunctional labor should be based primarily on aberrations in the graphic patterns of labor. This approach allows a continuous assessment of and provides an unequivocal language for communicating labor progress.18

Abnormal Labor

Latent Phase Dysfunction

Traditionally, a prolonged latent phase (Figures 44.3 and 44.4; Table 44.1) is diagnosed when the latent phase exceeds 20 hours in nulliparas or 14 hours in multiparas.3,12,19 Modern partograms allow much slower labor progression before 6-cm dilation and suggest that a prolonged latent phase should be judged based on individual patient management, rather than on a strict definition of the average duration of each phase.9

The diagnosis of a prolonged latent phase may lack precision because of the difficulty in ascertaining the exact time of labor onset. It is important to recognize that the latent phase may normally be quite long. A prolonged latent phase is not a predictor of active phase disorders, nor does it have a
strong association with cephalopelvic disproportion. It may sometimes occur as a result of a contractile abnormality that could become apparent later. Ignorance about the normal course of the latent phase may lead to unnecessary cesarean delivery under the erroneous assumption that continuous progress should be expected in all phases of labor or that very long labors are always abnormal.

Treatment of prolonged latent phase consists of maternal sedation or active efforts to stimulate uterine contractility (Figure 44.4). Stimulation with oxytocin effectively converts a prolonged latent phase to an active phase in 85% of cases, and this response generally occurs within 3 hours. A similar proportion of patients respond favorably to narcotic sedation (“therapeutic rest”). After a dose of morphine sulfate, the patient often sleeps for several hours and awakens in active phase labor. Having had a respite from many hours of painful contractions, she may be more eager and better prepared to cope with the physical and emotional demands of the active phase. Approximately 10% of women persist in a desultory latent phase after the effects of the narcotic have abated; in these cases, oxytocin stimulation is still necessary. Another advantage of therapeutic rest is that it allows the identification of the approximately 5% of women diagnosed with prolonged latent phase who are in false labor3,19; their contractions abate completely after narcotic treatment.

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Jun 19, 2022 | Posted by in OBSTETRICS | Comments Off on Normal and Abnormal Labor
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