Babies born by caesarean section may be more likely to be overweight or obese by the time they are adults, according to a large new study.
This isn't the first study to suggest that how a person begins life as a baby may predict obesity risk decades down the road.
While breast-feeding, appetite and other factors related to newborns have been linked to obesity risk, this study suggests the odds a child may develop a weight problem -- and related chronic disease risks -- could start the second a baby is born.
British researchers reviewed 15 studies on more than 38,000 babies in 10 countries who were tracked through adulthood.
They found the odds of being overweight or obese were 26 percent higher for babies born via C-section, compare to those birthed vaginally.
"This study shows that babies born by C-section are more likely to be overweight or obese later in life," study author Dr. Neena Modi, a professor of neonatal medicine at Imperial College London, said in a statement. "We now need to determine whether this is the result of the C-section, or if other reasons explain the association."
While the study did not look at why this might occur, the researchers noted that babies born by C-section have different gut bacteria, or microbiomes, than those delivered vaginally, which can play a major role in different ways throughout the body, including weight gain.
"We need to be exposed to the bacteria that come from mother which that baby may not be exposed to if it is born by C-section," Modi explained to CBS News' Lucy McDonald.
The compression that accompanies vaginal birth may also influence how certain genes are "switched on," which may have a long-term effect on metabolism, the researchers speculated.
Their study was published Feb. 26 in PLoS One.
Nearly one in three U.S. women give birth by C-section. The American College of Obstetricians and Gynecologists released new C-section guidelines this month to address concerns the procedures were overused in some women for reasons like convenience.
The guidelines urge doctors to allow women to progress longer during labor before ordering a C-section, following increasing evidence that showed early stages of labor may last longer than the 14-20 hour cutoff previously used to determine the need for surgery.
The definition of "active labor" -- when contractions become stronger and more frequent and hospitals typically admit women -- has also changed slightly after research found the cervix dilates slower than once thought.
But sometimes C-sections can be life-saving for both mom and baby, including in multiple births, when the baby isn't getting enough oxygen, is abnormally positioned in the birth canal, has problems with the umbilical cord and other medical concerns, according to the Mayo Clinic.