Rates of both Cesarean section birth and child obesity have soared in recent decades, and it turns out that the way children are brought into the world could have a big impact on their weight. Kids born by c-section have a different composition of bacteria in their guts than children born vaginally do, and scientists theorize this might make c-section babies more likely to be obese.

The weight difference between c-section and vaginally delivered babies showed up when researchers from Children’s Hospital Boston looked at 1,255 deliveries in eastern Massachusetts between 1999 and 2002. They measured and weighed the babies at birth, at 6 months, and at age 3.

One in four of the babies were delivered by c-section (a rate lower than the national average of one in three). Moms in the study who delivered by c-section tended to weigh more than those delivering vaginally, so the birthweight of their babies tended to be higher. They also breastfed their babies for a shorter period of time.

Even when these and other factors were taken into account, babies delivered by c-section were twice as likely to be obese by age 3 as those delivered vaginally. Just under 16% of the children delivered by c-section were obese, compared with only 7.5% of those born vaginally. Nationally, the obesity rate among kids ages 6-11 is nearly 20%, according to government figures.

Previous research has shown that children born by c-section have higher numbers of Firmicutes bacteria and lower numbers of Bacteroides bacteria in their guts than those born vaginally. This could increase the energy their bodies extract from food, and stimulate cells to boost insulin resistance, inflammation and fat deposits, say the study authors. They point out that the apparent link between cesarean delivery and obesity should prompt mothers to avoid c-sections unless they are medically necessary.

The research appears online in the Archives of Disease in Childhood, a BMJ publication.