NEW YORK (Reuters Health) - First-time mothers who are obese in their first trimester of pregnancy are nearly four times more likely to require a cesarean delivery than women who aren't overweight, a new study shows.

"First-time mothers should be given this information in advance of conception," Dr. Thomas Bergholt of the University of Copenhagen in Denmark and colleagues write in the American Journal of Obstetrics & Gynecology.

Heavier women are known to be at increased risk of a number of pregnancy complications, Bergholt and his team note. To better understand the influence of body mass index (BMI) on the likelihood of needing a cesarean delivery, the researchers looked at 4,341 women, all of whom were delivering their first child and went into spontaneous labor.

Among the women with BMIs below 25, meaning they were not overweight, 3.6% required C-section delivery, compared to 18.5% of the women with a BMI of 35 or above, which denotes obesity. C-section risk climbed steadily as BMI rose.

The researchers calculate that obese women have 3.8-times higher risk of undergoing a cesarean delivery as normal-weight women, after accounting for other factors that affect the risk.

There are a number of mechanisms by which excess weight could make it more difficult for women to deliver their infants vaginally, the researchers say. For example, heavier women tend to have bigger babies, while excess abdominal fat could interfere with the progression of labor.

Further studies should investigate whether interventions in early pregnancy might help prevent obese women from requiring C-sections, they conclude.