New data points to slower course of labor

Under the new analysis and in the larger, contemporary population of patients, the period of increasing slope was found to occur most commonly after 6 cm, not 4 cm. The slowest 5% of nulliparous women had cervical dilation of 0.4 cm per hour (with the median at 1.9 cm per hour), compared with 1.2 cm per hour (with a median of 3.0 cm per hour) as in the Friedman data.

Dr. Zhang’s study showed us that labor may take more than 6 hours to progress from 4 to 5 cm dilation, and more than 3 hours to progress from 5 to 6 cm dilation – a rate of progress that is significantly slower than what Dr. Friedman had described. The new data showed us, moreover, that from 4 cm-6 cm dilation, nulliparous and multiparous women progressed similarly slowly. Beyond 6 cm, multiparous women dilated more rapidly, with a steeper acceleration phase than previously described.

A consensus statement published in 2014 by the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM) on “Safe Prevention of the Primary Cesarean Delivery” encourages use of the Consortium data to revisit the definition of labor dystocia. While the data “do not directly address an optimal duration for the diagnosis of active-phase protraction or labor arrest, [they] do suggest that neither should be diagnosed before 6 cm dilation” (Obstet Gynecol. 2014 Mar;123[3]:693-711).

The ACOG-SMFM statement makes a series of recommendations for managing the first and second stages of labor, based not only on the Consortium data but on a broader literature review. It recommends that if mother and fetus appear well, cesarean delivery for active-phase arrest in the first stage of labor be reserved for women of at least 6 cm of dilation with ruptured membranes who fail to progress despite 4 hours of adequate uterine activity, or at least 6 hours of oxytocin administration with inadequate uterine activity and no cervical change.

Regarding the latent phase of labor, the statement says that most women with a prolonged latent phase ultimately will enter the active phase with expectant management. It advises that a prolonged latent phase (for example, greater than 20 hours in nulliparous women and greater than 14 hours in multiparous women) should not be an isolated indication for cesarean delivery.

The consensus statement also recognizes recent data showing that women who undergo labor induction have an even slower “normal” course of labor, particularly a longer latent phase, than women who labor spontaneously. A retrospective cohort study of more than 5,000 women, for instance, found that before 6 cm, women whose labor is induced can spend up to 10 hours to achieve each 1 cm of dilation (Obstet Gynecol. 2012 Jun;119[6]:1113-8).

As long as maternal and fetal status are reassuring, the statement says, cesarean deliveries for failed induction of labor in the latent phase can be avoided by allowing longer durations of the latent phase (up to 24 hours) and by requiring that oxytocin be administered for 12-18 hours after membrane rupture before deeming induction a failure.

Each of these described recommendations were graded in the ACOG-SMFM consensus document as “strong” recommendations with “moderate quality evidence.”

Examining our standards

Moving forward, we must further develop and define our thresholds for identifying who will most benefit from a cesarean delivery. We have many specific aspects of labor management to address as well, such as the optimal timing of artificial membrane rupture and the safety and efficacy of different oxytocin protocols. We may also want to revisit recommendations for serial cervical assessment, possibly adjusting the intervals given our understanding of the new labor curve.

Under the new labor paradigm, moreover, we must think not only about the clinical decisions we make at the bedside, but about the decisions we make early in the labor management process.

The timing of admission is one such decision. A statement published in 2012 on “Preventing the First Cesarean Delivery” by ACOG, SMFM, and the Eunice Kennedy Shriver National Institute of Child Health and Human Development advises us to avoid admittance of women during the early latent phase of labor (Obstet Gynecol. 2012 Nov;120[5]:1181-93).

It may even be advisable that we consider admittance at higher cervical dilation. A study published this year shows that women admitted at less than 6 cm of dilation had an increased risk of cesarean delivery, compared with women admitted at higher cervical dilation (Am J Perinatol. 2016 Jan;33[2]:188-94). We have more to learn, but certainly, given what we know now about labor progress and the start of active labor, the timing of admission is an important factor to consider.