SAN FRANCISCO - Overweight and obese pregnant women have a longer labor, especially if they have not had any previous births, according to a retrospective observational study conducted by the Consortium on Safe Labor.
Among the more than 100,000 women studied, the time required to progress from 4 cm to 10 cm of cervical dilatation increased steadily with body mass index. It was 2.9 hours (56%) longer for nulliparas and 1.4 hours (33%) longer for multiparas having the highest vs. lowest body mass index.
The duration of the second stage of labor also increased with BMI among the nulliparas, particularly if they had an epidural, according to results reported at the annual meeting of the Society for Maternal-Fetal Medicine.
"This information highlights the concept that contemporary labor practices should take into account the changing profiles of our obstetrical population, particularly increasing BMI," commented principal investigator Dr. Michelle A. Kominiarek. "Allowing for a slower progression of labor for obese gravidas prior to intervening with a cesarean has the important potential of decreasing the cesarean delivery rate."
Excess weight during pregnancy has been linked to both dysfunctional labor and cesareans, she observed. "The explanation for these findings is not known, but greater fetal size, soft tissue obstruction to labor, poor uterine contractility, more frequent inductions as a result of pregnancy complications, and caregiver biases have been proposed."
The investigators undertook the study "to describe labor patterns in a contemporary obstetrical population, one with greater inductions, greater epidurals, fewer operative deliveries, and presumably a greater maternal weight," explained Dr. Kominiarek of the University of Illinois at Chicago.
They analyzed records in an obstetric database that captured data from 12 U.S. clinical centers for the years 2002-2008. Results were based on 118,978 women who had labor at term; had a singleton, live-born infant with cephalic presentation; and had not had a previous cesarean.
The women were divided into five categories of BMI at the time of admission (less than 25.0 kg/m2, 25.0-29.9, 30.0-34.9, 35.0-39.9, and 40.0 or greater).
The average BMI was 30.5 for the entire cohort, and slightly more than 7% of the women had a BMI in the highest category, Dr. Kominiarek reported.
Results showed that as BMI increased, cervical dilation proceeded more slowly, so that women in each BMI category had a distinct labor curve when dilation was plotted against time.
Among nulliparas, there were no clear inflections in these curves to indicate when labor transitioned from the latent to the active phase.
After adjustment for potential confounders (age, race, gestational age, diabetes, induction of labor, augmentation of labor, epidural use, and operative vaginal deliveries), the median time to progress from 4 to 10 cm of cervical dilatation was 5.2 hours for nulliparas in the lowest BMI category, compared with 8.1 hours for those in the highest (P less than .001).
The findings were similar in analyses that looked at each additional centimeter of cervical dilatation. The prolongation with increasing BMI was most evident early in labor, going from 3 to 4 cm and from 4 to 5 cm.
BMI was also associated with a slowing of the duration of the second stage of labor for nulliparas who had an epidural, but less so for those who did not, according to Dr. Kominiarek.
Among multiparas, the labor curves did show inflection points, with a rapid acceleration of cervical dilation indicating entry into active labor. However, the higher a woman’s BMI, the longer it took to get to that point.
The adjusted median time to progress from 4 to 10 cm of cervical dilatation was 4.3 hours for multiparas in the lowest BMI category, compared with 5.7 hours for those in the highest (P less than .001).
The findings here were also similar in analyses that looked at each additional centimeter of cervical dilation, with BMI having greatest impact on the progression from 4 to 5 cm. But the associations were mostly not significant.
Higher BMI did not slow the duration of the second stage of labor among multiparas, regardless of whether they had an epidural.
The reason for the prolongation of labor with increasing BMI is not yet clear, according to Dr. Kominiarek. But she speculated that some of the aforementioned factors, such as weight-induced alterations in physiology or comorbidities, may affect the pace of labor.
Dr. Kominiarek did not report any relevant conflicts of interest.