Pregnant women who put on more pounds than generally recommended are more likely to undergo a Cesarean delivery, according to a new study.

Researchers found that among nearly 9,800 first-time mothers, those who gained more weight than U.S. guidelines suggest were more likely to need a C-section, even when the baby was not large.

That point is important because it’s known that heavy birthweights - 8-plus pounds or more - increase the odds of needing a C-section. The new study suggests that something about excess maternal weight gain itself makes a Cesarean more likely, according to Dr. Naomi E. Stotland of the University of California, San Francisco.

She and her colleagues report the findings in the journal Obstetrics & Gynecology.

In the U.S., the Institute of Medicine (IOM) recommends that for optimal mother and newborn health, normal-weight women should gain 25 to 35 pounds during pregnancy, while overweight women should put on no more than 25 pounds.

However, it’s known that many women fall outside of these ranges, Stotland told Reuters Health. In her team’s study, 48 percent of the women gained more than the recommended weight, while 17 percent gained less.

Overall, women who gained excessive weight were 40 percent more likely to have a C-section than those who stayed within the recommended range. Even when the researchers factored in birthweight, the link between maternal weight gain and Cesarean risk remained.

According to Stotland, the findings lend additional support to the IOM guidelines. She and her colleagues estimate that of the 288,000 C-sections performed each year on first-time mothers in the U.S., 64,000 would be prevented if no woman exceeded the recommended weight gain limits.

Exactly why excess weight gain may lead to C-section in some cases is unclear. Stotland speculated that doctors may sometimes choose to perform the procedure when a heavier woman is having a hard time during labor.

Or, she said, hormone levels - which are influenced by fat tissue - may come into play, as might excess soft tissue, which could hinder vaginal delivery.

Stotland also noted that it was only among initially normal-weight women that excess weight gain raised the odds of C-section. Overweight women who gained more than 25 pounds had a Cesarean rate similar to that of women who fell within the IOM guidelines.

On the other hand, overweight women who gained less than the IOM recommended range seemed to have a lower risk of C-section, though this finding was not significant in statistical terms. ]

“We’re not suggesting that women should gain less” than the IOM recommends, Stotland said.

However, she added that more research is needed to determine how many pounds heavy women should ideally put on during pregnancy. Right now, Stotland noted, there is no upper limit set for women who are obese.

SOURCE: Obstetrics & Gynecology, October 2004.