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.

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

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.

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