Normal labor occurs in four stages. The first stage is cervical dilation, the second is delivery of the fetus, the third is placental expulsion, and the fourth is recovery. The first stage is subdivided into latent and active phases. The latent phase, the earlier of the two, is characterized by regular contractions perceived by the mother and relatively slow cervical dilation. According to Friedman, who began analyzing normal labor progress as early as 1954, the mean duration of the latent phase in nulliparous women is 8.6 hours, and 5.3 hours in multiparous women. Using two standard deviations as the outer limit of normal, the latent phase should be less than 20 hours in nulliparae and less than 14 hours in multiparae. Friedman’s work has been one of the foundations of labor and delivery management for decades, although some authors have questioned the definition and even the existence of the latent phase.

Active labor, the second phase of the first stage, is characterized by an accelerated rate of cervical dilation. Technically speaking, the precise onset of active labor can only be determined retrospectively after graphically plotting the labor curve for any given patient. Clinically, most women transition from the latent to the active phase when the cervix is between 3 and 4 cm dilated, although in one series 30% of women reached 5 cm of cervical dilation prior to entering active labor. Thus, the combination of regular uterine contractions and cervical dilation between 3 and 5 cm is a useful clinical marker for active labor. In Friedman’s graphical analysis of labor, the mean duration of the active phase among nulliparous women was 4.9 hours but a wide range was observed. Thus, the rate of cervical dilation in active labor varies, but should proceed at a minimum of 1.2 cm/h for nulliparae and 1.5 cm/h for multiparae. Gravid women who have entered into active labor and who dilate at a slower rate are said to have protracted labor or labor dystocia, whereas those whose cervix is unchanged over a period of 2 hours are said to have an arrest of dilation.

The second stage of labor begins when cervical dilation is complete and ends with the expulsion of the fetus. Although the cardinal fetal movements in labor occur primarily in the second stage, descent of the fetal head is usually noted beginning around 7–8 cm of dilation. Thus, fetal descent bridges the first two stages of labor. The rate of this descent was also quantified in Friedman’s work and proceeds at least 1 cm/h in nulliparae and 2 cm/h in multiparae. Arrest of descent is diagnosed if the fetus does not advance in station over a period of 1 hour. The total duration of the second stage of labor should be less than 2 hours in nulliparae and less than an hour in multiparae. These limits are extended by an hour if regional anesthesia is used.

The origin of these guidelines is difficult to determine, but they were already established by the time the first edition of Williams’ Obstetrics was published in 1903 and currently remain widely accepted. In fact, the American College of Obstetricians and Gynecologists (ACOG) uses these same parameters to define both prolonged second stage as well as second-stage arrest. Strict adherence to these limits may lead to unnecessary cesarean or forceps deliveries. More recent data suggest that prolonged second stages in closely monitored patients, even in cases where the second stage exceeds 6 hours, are not associated with adverse outcomes such as low Apgar scores, NICU admissions, neonatal seizures or neonatal death. Being mindful of the usual limits for the second stage of labor, the provider nevertheless has some leeway to exercise judgment regarding operative delivery versus continued observation, depending on the specific clinical situation, in particular continued progress in descent. In any event, if one does decide to allow the second stage to continue beyond the usual parameters, thorough documentation of the rationale behind this decision is critical.

In labors which are progressing slowly, a combination of several variables may be involved. These are commonly summarized as “the passage, the passenger, and the powers.” Although these factors are considered separately in the ensuing discussion, they are probably not always independent. For example, inadequate uterine activity may result from an excessively large or malpositioned fetus attempting to pass through a narrow pelvis.