The results don't prove that c-sections cause kids to put on weight, but Dr. Jianmeng Liu, one of the authors of the study and a professor at Peking University Health Science Center in China, said the link between the delivery and obesity is important to keep in mind.

"The potential health burden of obesity and other diseases associated with c-section births should not be neglected, even if its impact is modest, particularly given" how often births happen that way, Liu told Reuters Health in an email.

Previous research has tied c-sections to a variety of untoward health outcomes in children, including asthma, allergies and diabetes (see Reuters Health reports of February 5, 2009 here: reut.rs/js7tcW and September 18, 2008 here: reut.rs/m5Kpji).

Liu said that the relationship between the type of delivery and obesity among kids hasn't been as clear (see Reuters Health reports of January 30, 2012 here: reut.rs/xxjBgo and May 12, 2011 here: reut.rs/mv2kS5).

The research team collected the results from nine studies that included more than 200,000 people.

People were 33 percent more likely to be overweight or obese if they were born by c-section, researchers report in the International Journal of Obesity.

Nearly 70 percent of adults in the U.S. are overweight or obese. A 33 percent increase from that number would mean that 93 percent would be heavy.

The risk for childhood obesity in particular was somewhat higher - about a 40 percent increase over kids born vaginally.

Nearly one in five kids aged six to 11 is obese in the U.S., according to the Centers for Disease Control and Prevention.

Liu said the increase in risk was modest, but that it persists into adulthood. When the researchers looked just at the studies on adults, they found that those who were born surgically were 50 percent more likely to be obese than those who were born vaginally.

WHY THE LINK?

It's not clear why c-section births are tied to a better chance of being heavy.

One possibility relates to the bacteria babies are exposed to when they are delivered vaginally, which might affect the way they process and store food, said Liu.

Additionally, Liu added, researchers have suggested that c-sections are linked with a lower concentration in the umbilical cord of a hormone important in regulating weight and with a reduced rate of breastfeeding, "both of which are reported to be associated with an increased risk of later obesity."

Babies who are larger than normal are also more likely to be born via cesarean, but most of the studies Liu's team analyzed took into account birth weight.

Cesareans have become increasingly popular, and in the U.S. now one in four babies is born through a c-section.

Liu said there's been concern that some of these are unnecessary, and given the potential negative impacts on children the unneeded ones should be curbed.

"In clinical practice, (the) potential adverse impact of c-section should be considered by medical staff, and non-medically indicated elective c-section should be somewhat avoided, where possible," Liu said.

SOURCE: bit.ly/XcjOMh International Journal of Obesity, online December 4, 2012.

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