The course of normal human labor and delivery comprises a complex relationship between several dynamic parameters, including uterine contractions, cervical dilation, fetal descent, and elapsed time. Once the diagnosis of labor is made correctly, one can apply empiric models of progress in labor to identify labor abnormalities and apply effective interventions.

The onset of labor in humans occurs around 280 days, or 40 weeks, from the first day of a patient’s last menstrual period (LMP). Because the estimated date of confinement (EDC), or “due date,” is associated with much anticipation and planning on the part of patients, care must be taken to educate the patient about the uncertainties inherent in setting the EDC as well as to ensure that the assignment of the due date is based on accurate medical data, insofar as this is possible. Individual variation accounts for a range for the onset of labor that spans 2 weeks on either side of the best estimated EDC; spontaneous labor between 38 and 42 weeks is considered normal. It is then the responsibility of the health care provider, when estimating a date of confinement, to take an accurate menstrual and contraceptive history in order to avoid assigning an incorrect date. If the patient’s cycle length is anything other than 28 days, the EDC must be adjusted accordingly, as much of the variation in cycle length is associated with the follicular, or preovulatory, phase. If conception occurs while the patient is using, or had recently been using, hormonal contraception, the date of ovulation may again be something other than 14 days after the LMP. If so, early ultrasound may be warranted in order to date the pregnancy more accurately and avoid mis-timing of medical interventions. While such attention to detail may seem insignificant at the time, many interventions including unnecessary tocolysis and unindicated induction of labor may result from an inaccurately dated pregnancy.

The physiology of normal labor in humans remains incompletely understood. Evidence from sheep models suggests that the causative event in labor onset is a fall in maternal serum progesterone, concomitant with a rise in estrogens, all triggered by fetal adrenal cortisol production. However, a dramatic decrease in serum progesterone at term is not seen in humans, and an intact fetal hypothalamic-pituitary-adrenal axis does not seem necessary for labor to occur, as observed in pregnancies complicated by fetal anencephaly where the average delivery date is 39 6/7 weeks. Research in murine models suggests a role for prostaglandin synthesis in the onset of labor, although such data are again limited by interspecies differences. It is likely that the human uterus, a muscular organ with significant resting tone outside of pregnancy, is under negative inhibition during the bulk of pregnancy, and only near term is that negative inhibition lifted, thus enabling coordinated uterine contractions to occur. Because
of the relatively poor understanding of the physiology of human labor, effective treatments for preterm labor and for induction of labor have remained elusive.

Clinically recognizable labor is typically divided into three stages, each with statistically derived normative rates and durations. Many of these labor values were elucidated by Emanuel Friedman, who in the 1950s published his studies of hundreds of normal and abnormal labors and plotted cervical dilation and fetal descent against time. The resulting graphic labor curve was used to recognize individual labor patterns that deviated from normal and to guide the nature and timing of interventions. A more recent evaluation of labor and delivery patterns takes into account changes in both medical management of labor (including higher induction rates, more use of oxytocin and regional anesthesia, and higher rates of continuous fetal monitoring) and in patient characteristics (including increased body mass index [BMI] and decreased smoking rates, both of which have contributed to an increase in fetal size) and suggests a significantly longer duration of the active phase of labor among the contemporary population. Regardless, the very practice of tracking labor in a formal fashion appears to improve labor outcome. In a World Health Organization study of 35,484 women, use of a “labor curve” or partogram (Fig. 2.1) and an agreed on labor management protocol was associated with a reduction in the percentage of prolonged labors, the proportion of labors requiring augmentation, and postpartum sepsis.

The first stage of labor consists of the time between the onset of regular contractions associated with cervical change and the occurrence of complete cervical dilation. The first stage is further divided into latent and active phases. Although the distinction between the two phases can be difficult to make, the latent phase of labor is characterized by a slower rate of cervical dilation despite strong, regular uterine contractions. The latent phase can normally last up to 14 hours in multigravid patients and up to 20 hours in nulligravidas. In the active phase of labor, there is a more rapid change in cervical dilation. Patients may move extremely rapidly through active labor, although the lower limit of normal for cervical change is about 1 cm per hour for nulliparous women.

In the majority of patients, the transition between the latent and active phases occurs at some time between 3 and 5 cm of cervical dilation, although it is possible, particularly in multigravid patients, to see a patient who is 5-cm dilated and still in the latent phase of labor. It is also critical for the clinician to accurately distinguish between latent phase labor, during which incremental cervical change is occurring (although slowly), and dysfunctional uterine contractions, a condition characterized by no change in cervical dilation despite strong, painful uterine contractions. Such dysfunctional contractions do not constitute labor, and treating them as such may lead to unnecessary intervention.

The second stage of labor is defined as the interval between complete cervical dilation and delivery of the baby. This stage is characterized by descent of the fetal presenting part; maternal sensation of pelvic pressure as this descent progresses; and maternal expulsive efforts, which in concert with uterine contractions effect delivery of the baby. The duration of the second stage varies with parity, ethnicity, fetal size, and the presence or absence of regional anesthesia and can range from only minutes to as much as 3 hours.

Finally, the third stage of labor comprises that time period between delivery of the baby and delivery of the placenta and may take up to 30 minutes, although usually is much shorter.

Human labor differs from that of other mammals, not only with regard to physiology but also in the way in which the
fetus moves through and out of the birth canal. The mechanism of human labor is complicated by two main evolutionary changes: increased brain size and changes in pelvic shape due to bipedal posture. Both present challenges to the “fit” between the fetal skull and the maternal pelvic outlet. Whereas labor complications in other mammals are mostly related to malpresentations, labor dystocia in humans may occur simply due to fetal head position or subtle differences in the shape of the maternal pelvis. Therefore, it is critical for the obstetrician to understand the anatomy of the pelvis as well as how to assess the presentation, lie, and position of the fetus.

Clinical assessment of the pelvis involves manual evaluation of the pelvic inlet, midpelvis, and outlet (Figs. 2.2, 2.3).

Consideration of these measurements allows assignment to one of the various pelvic types and thus an appreciation of how and where labor may be stalled if the pelvis is not favorable for childbirth. Careful evaluation of the midpelvis is most important, as those women found to have a contracted midpelvis are poor candidates for forceps-assisted vaginal delivery. However, because the fetal skull has the ability to mold, and because overall fetal size is variable, borderline pelvimetry is not a contraindication to a trial of labor.

Clinicians who provide care for women in the third trimester of pregnancy should assess the orientation of the fetus at each visit. Early detection of abnormal fetal orientation can increase the success of interventions to correct this; for example, the chance of successful external cephalic version of a breech fetus is greater if the version is performed prior to the onset of labor.

The fetal lie is the relationship between the sagittal plane of the fetus and the mother. The vast majority of patients in labor have a longitudinal fetal lie, although risk factors including multiparity and uterine or fetal anomalies may increase the rate of transverse or oblique lie. Fetal presentation refers to the part of the fetus that is closest to the pelvic inlet. Most often, the fetus is in cephalic presentation,
and of those, the majority are in a vertex (posterior fontanel as the presenting landmark) presentation. Other presentations include brow and face. Breech presentation is classified into several subcategories: complete (hips and knees flexed), frank (hips flexed, knees extended), and incomplete or footling (one or both lower extremities presenting). Finally, fetal position describes the relationship of a presenting part to the maternal pelvis. For purposes of describing fetal position, the point of reference in a vertex presentation is the occiput; for a breech, it is the sacrum; and in face presentations, it is the chin (or mentum). The reference point is described in its relationship to the maternal pelvis. Thus, with a vertex presentation, the occiput on the maternal left side of the pelvis, and the fetal sagittal suture transverse in the pelvis, the position is left occiput transverse, abbreviated as LOT.

The clinician can often determine fetal lie and presentation by manual palpation of the gravid uterus. This process was formalized in four discrete maneuvers described by Leopold in the late 19th century (Fig. 2.4). Fetal position generally cannot be determined by external examination but rather by vaginal examination and direct palpation of the fetus during active labor or by ultrasound investigation.

From the perspective of the fetus, labor involves movement progressively downward through the pelvis by the following cardinal movements, described for a vertex presentation (Fig. 2.5).