NEW YORK - Kids born by Cesarean section are no more likely to become obese than if they are born vaginally, a new study concludes.
Past research from Brazil had found a link between excessive poundage and C-sections, leading some scientists to suggest that not being exposed to bacteria from the birth canal could make babies fatter.
But according to the latest findings, that doesn't appear to be the case.
"We thought from the beginning that probably what happened with the previous study is that they didn't adjust for all of the confounders," said Fernando Barros of the Catholic University of Pelotas. "If a mother gives birth by C-section, she's different than a mother who has a vaginal birth."
For the new research, Barros and his colleagues used data on three groups of several thousand people born in Southern Brazil in 1982, 1993 or 2004.
Researchers contacted the kids at different ages until the oldest had turned 23. Those born by C-section were more likely to be heavy, with obesity rates between 9% and 16%, compared to rates of 7% to 10% among kids born vaginally.
However, that difference vanished once the researchers accounted for factors that could have influenced the results such as family income, birth weight, schooling and the mother's weight, height, age and smoking habits.
"When you factor in all of these other factors, the relationship between obesity and Cesarean sections disappears," said Barros, whose findings are published in the American Journal of Clinical Nutrition.
The earlier Brazilian study left out many of those factors, including maternal height and weight, Barros' team writes in its report.
"The most simple explanation would be that more obese women require more Cesarean sections than lean women… and it's really not the C-section itself," said Dr. David Ludwig, director of the Optimal Weight for Life Clinic at Children's Hospital Boston, who wasn't involved in the study.
The new research is of particular interest in Brazil, because in 2009 more than half of the babies there were born by C-section. In the US, the number has been on the rise for years and is now over 30%.
Some believe that C-section babies are different because they are not exposed to bacteria in the birth canal like babies born vaginally. The theory is part of the hygiene hypothesis, which suggests a person's immune system develops differently when they're not exposed to beneficial bacteria early in life.
"We're not saying this hypothesis is not interesting. It is. We're just saying, right now, without data, we cannot confirm the finding," said Barros.
He cautioned that people in his study had only been followed until early adulthood, so he cannot say if there is a potential association later in life.
Ludwig told Reuters Health that things like a pregnant woman's diet and smoking habits and whether or not she has diabetes might influence a developing fetus.
Both Ludwig and Barros said women should avoid medically unnecessary C-sections, even if they don't raise the chances of having obese kids, because they carry other risks.