New data points to slower course of labor

Only recently has evidence emerged that challenges our long-held understanding of “normal” and “abnormal” labor. We now know there is a much wider range of normal labor progress in women who go on to have good labor outcomes. We have a new labor curve to guide us – one that shows us, for example, that active labor occurs most commonly after 6 cm dilation rather than 4 cm as we’d previously thought.

By appreciating this new labor paradigm, we can potentially have a significant impact on the cesarean rate in the United States. While our use of the older labor curve is not the only reason for the rise in cesarean deliveries over the last 30 years, it very likely has played a role. A study published in 2011 of more than 32,000 live births at a major academic hospital demonstrated that one of the most common reasons for primary cesarean is abnormal labor or arrest (Obstet Gynecol. 2011 Jul;118[1]:29-38).

Dr. Alison G. Cahill

Another study by the Consortium on Safe Labor – an analysis of labor and delivery information from more than 228,000 women across the United States – showed that half of the cesarean deliveries performed for dystocia in women undergoing labor induction were performed before 6 cm of cervical dilation and relatively soon after the previous cervical examination (Am J Obstet Gynecol. 2010 Oct; 203[4]: 326.e1–326.e10).

Our new labor paradigm brings to the forefront a host of new issues and questions about how we can best manage labor to optimize outcomes. In a way, recent discoveries about labor progress have highlighted a dearth of evidence and made “old” issues in labor management seem new and urgent.

As we strive to learn more, however, we are challenged to change our practices and behavior at the bedside with the evidence we currently have. By appreciating both the new labor curve and our current understanding of how labor induction, obesity, and other patient characteristics and clinical conditions can affect labor progress, we can expect that many women will simply progress much more slowly than was historically expected.

As long as we have indications of the well-being of the baby and the well-being of the mother, a slower but progressive labor in the first stage should not prompt us to intervene. We should no longer apply the standards of active-phase progress – standards that have traditionally driven our diagnoses of labor dystocia – until the patient has achieved 6 cm of dilation.

The labor curve that had shaped our thinking about normal and abnormal labor progress until recently was developed by Dr. Emanuel Friedman. Based on findings from a prospective cohort study of 500 nulliparous women, Dr. Friedman plotted labor progress with centimeters of cervical dilation on the Y-axis and time on the X-axis, and divided labor into several stages and phases. In this curve, the rate of change of cervical dilation over time started increasing significantly at 4 cm; this period of increasing slope defined the active phase of labor.

Abnormal labor progress in the active phase was then defined, based on the 95th percentile, as cervical dilation of less than 1.2 cm per hour for nulliparous women and less than 1.5 cm per hour for multiparous women. Based on Dr. Friedman’s work, a woman was deemed to be in active-phase arrest when she had no cervical changes for 2 hours or more while having adequate uterine contractions and cervical dilation of at least 4 cm. These concepts came to govern labor management.

The paradigm shifted when the Consortium on Safe Labor reported in 2010 on a retrospective cohort study of more than 62,000 women at 19 U.S. hospitals. The women had a singleton term gestation, spontaneous labor, vertex presentation, vaginal delivery, and a normal perinatal outcome. In their analysis of labor and delivery information, Dr. Jun Zhang of the National Institutes of Health’s Eunice Kennedy Shriver National Institute of Child Health and Human Development and his colleagues accounted for the fact that the exact times of cervical change are unknown.

They used modern statistical methods and analytical tools that took into account the specific nature of cervical dilation data – that cervical measurements are interval-censored (we never know the exact time when a woman’s cervix changes) and that multiple exams of the cervix in the same patient are not independent (Obstet Gynecol. 2010 Dec;116[6]:1281-7).

The methodology used in the Consortium study accounted for both the interval-censored and repeated-measures nature of cervical dilation data. It thus addressed analytical flaws in the previous approach to labor data, which was purely descriptive of the exam findings and did not consider the nature of the data itself.