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  • What is the definition of latent vs. active labor?

  • What are the best practices for vaginal delivery?

  • How is active management of labor achieved?

  • How is labor arrest diagnosed?

  • How do I avoid a primary cesarean delivery?

CASE 37-1

A 32 y.o. primiparous woman presents to L&D triage complaining of painful, regular contractions. She is normotensive, afebrile, and amniotic membranes are intact. She states that she is able to cope with her pain 7/10 at present. Her fetus is a Category 1 tracing, and tocometry shows contractions every 4 to 5 minutes apart. On sterile vaginal exam, she is 4 cm dilated, 90% effaced, and -1 station.

Labor is defined as uterine contractions with a frequency and intensity strong enough to cause cervical dilation, cervical effacement, and ultimately descent of the fetal head into the maternal pelvis. From the Latin ob and stare is derived the term obstetrics, which means “to stand by.” Thus as obstetricians we are standing by the expectation and the hope, the contractions and the pain, and ultimately the descent of the fetal head into the maternal pelvis—waiting for the birth of new life.


The physiology of parturition is an orchestrated event involving many players—the myometrium, the decidua, and the uterine cervix. Throughout pregnancy, true labor is held in check by inhibitors such as progesterone, prostaglandin I-2, relaxin, and nitrous oxide. As the uterus readies for labor over the course of days and weeks, the increased expression of prostaglandin and oxytocin receptors is upregulated, in addition to an increase in myometrial gap junction formation. Following the onset of early labor, prostaglandins E2/F2-alpha, estrogen, and oxytocin coordinate to stimulate the myometrium, providing regular, rhythmic uterine contractions. Recent studies have posited that the surfactant molecule in the lungs of the fetus may play a role in parturition.1 The pear shape of the uterus, as well as the gynecoid shape of the maternal pelvic bones, positions the fetus in the vertex presentation in nearly 97% of deliveries. The mechanical steps of the fetus in its passage through the birth canal are engagement, descent, flexion, internal rotation (most commonly, this is where the fetus rotates to the occiput-anterior position), extension, external rotation, and expulsion (Fig. 37-1). Station of the fetal head refers to where the biparietal diameter is in relation to the ischial spines of the maternal pelvis. Engagement of the fetal head is confirmed when the biparietal diameter of the vertex is at or past the level of the pelvic inlet. While engagement is an essential element of the cardinal movements, it can, and often does, happen before labor starts.

FIGURE 37-1.

Cardinal movements of L&D. (Reproduced with permission from Cunningham FG, Leveno KJ, Bloom SL, et al: Williams Obstetrics, 25th ed. New York, NY: McGraw-Hill Education, Inc; 2014).

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