More babies born via C-section grow up to be heavy kids and teens than those delivered vaginally, according to a new study of more than 10,000 UK infants.
Eleven-year-olds delivered by C-section, for example, were 83 percent more likely to be overweight or obese than their vaginally-born peers once other related factors — such as their mother's weight and how long they were breastfed — were taken into account.
The findings are in line with a recent review of nine earlier studies that also found a link between C-sections and childhood obesity.
With C-sections, "there may be long-term consequences to children that we don't know about," said Jan Blustein, M.D., who led the new study at the New York University School of Medicine.
The rate of C-sections in the U.S. has been rising, leading to concerns about possible complications for mothers and babies. According to the Centers for Disease Control and Prevention, C-sections accounted for almost 1 in 3 births in 2010 — up from 1 in 5 in 1996.
For women, the procedure increases the chance of bowel or bladder injuries as well as future pregnancy complications.
Dr. Blustein said the size of the obesity risk for kids is "not great," and shouldn't come into play for women who need a C-section for medical reasons.
But, "a woman who's considering C-section electively should probably know about those risks," she said.
The researchers analyzed data from babies born in Avon, UK in 1991 and 1992 who were followed through age 15. Just over 9 percent of the infants were delivered via C-section. On average, kids delivered by C-section were born slightly smaller — by less than two ounces — than those who went through vaginal birth.
Starting at six weeks of age, however, C-section babies were consistently heavier than vaginally-born infants at almost all check-ins. That link was especially strong among children born to overweight mothers, Dr. Blustein and her colleagues report in the International Journal of Obesity.
Across the whole study group of children, rates of overweight and obesity ranged from 31 percent at age three to 17 percent at ages seven and 15.
Dr. Blustein said studies haven't been able to prove whether C-section, itself, is a reason some babies tend to gain more weight.
If it is, she speculates, it might have something to do with C-section babies missing out on important exposures to friendly bacteria during the trip through the birth canal.
"Generally, the early colonization and establishment of the intestine with bacteria seems very important. Yet, much more work is needed before we can explain the mechanisms of the early bacterial colonization," Teresa Ajslev, from the Institute of Preventive Medicine in Frederiksberg, Denmark, told Reuters Health in an email.
For example, there may be a specific type of bacteria that's protective, said Ajslev, a researcher and PhD student who has studied pregnancy-related impacts on childhood weight but wasn't involved in the new report.
Or bacteria imbalances could more generally disrupt intestinal function in a way that promotes obesity.
Either way, if the exact cause could be identified, it might be possible to give C-section babies doses of the missing gut bugs to restore balance.
But it's also possible bacteria have nothing to do with the obesity link to C-section births.
"The other possibilities are [that] these are children that would have been heavier anyway," Dr. Blustein said.
"Being heavy as a woman is a risk factor for C-section, so that's the problem with trying to figure out whether this is real or if it's simply a matter of selection," since overweight parents are more likely to have overweight children.
Her study was able to take a mother's weight into account, and did find the link between C-section births and child obesity was "weak" among kids born to normal-weight mothers.
But there could be other unmeasured factors that help explain the overall link between delivery method and a child's weight.
"This certainly is not the last word," Dr. Blustein said.
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