SAN FRANCISCO – A threshold of 6-cm cervical dilation is more accurate than the conventional 4 cm to determine when a woman enters the active phase of labor, a reevaluation of evidence suggests.

The historical evidence behind the commonly used assumption that 4-cm dilation signals the start of active labor contains methodological flaws, doesn’t match today’s population of pregnant women, and is contradicted by more recent studies supporting the 6-cm threshold, Tekoa King, C.N.M, Ph.D., said at a meeting on antepartum and intrapartum management sponsored by the University of California, San Francisco.

Using the 4-cm threshold, a woman "may just be in the normal latent phase of labor," she said.

Switching to the 6-cm threshold should delay or reduce the use of epidural anesthesia and might lower the high rate of cesarean sections in the United States. "Six centimeters is the new four centimeters," said Dr. King, a certified nurse-midwife and clinical professor of nursing at the university.

The 4-cm threshold for active labor arose out of two studies in the 1950s by Friedman et al. One study plotted the course of labor in centimeters of dilation over time in 500 nulliparous pregnant women. The same investigators profiled 200 women who were considered in the 1950s to have "ideal labor" – meaning term, vertex, singleton pregnancies – but the women commonly received considerable amounts of morphine and may or may not have had instrumented deliveries using outlet forceps. "They wouldn’t be what we would consider ideal today," Dr. King said.

Comparing the two cohorts, investigators in the 1950s found that the "ideal" group had shorter labors. They concluded that women who had been contracting more than 24 hours had a prolonged latent phase, and that the slowest rate of dilation in the active phase of labor was 1.2 cm/hr. Thus was born the "Friedman curve" that underpinned the decades-long dogma that women need to dilate about 1 cm/hr in the active phase of labor.

More recently, other data show that "the Friedman curve was really codified by the way we examined women every 2 hours. If you examine them more frequently, you’re going to get a different curve. Probably what we should be doing is examining them a lot less frequently until they’re 6 cm, and perhaps a little more frequently from 6 cm to complete" dilation, depending on whether or not there are other indications for examination, she said.

Dr. Tekoa King

A 2002 study by Zhang et al. of 1,329 nulliparous, term, singleton, vertex pregnancies with normal-weight babies, spontaneous onset of labor, and vaginal delivery used a different approach from the Friedman studies. It measured the time between each centimeter change in dilation instead of the time it took to go from 4-cm to 10-cm dilation and then calculating an interval average. The newer study found that it was common to have very slow progress before 7 cm, there was no deceleration phase, and the slowest but still normal rate of cervical dilation was less than 1 cm/hr, with a wide range of variability (Am. J. Obstet. Gynecol. 2002;187:824-8).

To go from 6-cm to 7-cm dilation, for example, took little more than half an hour on average, but it ranged from 0.2 hours to more than 2 hours. "This makes sense," Dr. King said. "What’s really happening between 6 and 7 or 7 and 8 cm? Internal rotation. We often forget that internal rotation and descent station are progress. We just get ourselves fixated on cervical dilation" and end up performing a cesarean section in women for "arrested labor" at a time when they’re having normal progress.

The same investigators followed that with a 2010 study analyzing data on more than 50,000 singleton, vertex pregnancies with spontaneous onset of labor, vaginal delivery and "normal outcome." Again, they used the "repeated measures" approach to estimate the labor curves and to "redefine normal," Dr. King said. They found that the median rate of change was about 2 cm/hr and the slowest rate of normal change was 0.4 cm/hr (Obstet. Gynecol. 2010;116:1281-7).

On Friedman’s curve, what he called the point of inflection (or the beginning of the active phase of labor) was at 4-cm cervical dilation. But Zhang’s curve suggests that the point of inflection when labor starts to progress faster is at 6-cm dilation. "That’s our real world today, and here we are, using 4 cm. Basically, we’re treating women who are in the latent phase of labor as though they were active," she said.

The other insight from the work of Zhang et al. is that dilation progresses faster as the cervix becomes more dilated, not at a steady rate of 1 cm/hr.