Infants who are delivered through Cesarean section are twice as likely to become obese by the age of three as those delivered vaginally, researchers warned.

Researchers said that giving birth via C-section might affect the composition of bacteria in the infant's gut, which in turn affects the way food is digested. 

The study, published Wednesday in the Archives of Disease in Childhood, was based on 1,255 mother and child pairs who had attended eight outpatient maternity services in eastern Massachusetts, USA between 1999 and 2002.

The mothers had joined the study before 22 weeks of pregnancy and their babies were measured and weighed at birth, at six months, and then at the age of three.

Out of all the deliveries about 23 percent were delivered via C-section and the rest were vaginal deliveries.

The findings suggest that there is a link between body mass, skin thickness and how a child was delivered. 

Researchers found that mothers who delivered by C-section tended to weigh more than those delivering traditionally, and the babies who were delivered via Cesarean also tended to have higher birth weights. 

Researchers analyzed the infants' body mass index using a skin-fold test when the children were 3 years old and found that 15.7 percent of the babies delivered by C-section were obese compared to 7.5 percent of those delivered vaginally.

They noted that children delivered by C-section were also on average heavier and had more body fat.

Researchers had accounted for variables such as the mother's weight, babies' size and length of time mother breastfed their babies. 

They found that mothers who delivered by C-section tended to weigh more than those delivering traditionally, and the babies who were delivered via Cesarean also tended to have higher birth weights. Mothers who had C-sections also didn't breastfeed their babies as long as mothers who gave a traditional birth.

Researchers also found that babies born via C-section also have higher numbers of Firmicutes bacteria and lower numbers of Bacteroides bacteria in their guts. They suggested that C-sections might change the way babies are exposed to and obtain colonies of key gut flora from their mothers during birth.

Previous studies have also found that obese people have higher levels of Firmicutes bacteria, and that the gut bacteria may be influencing the development of by taking more energy from a person's diet, and by stimulating cells to boost insulin resistance, inflammation, and fat deposits.

"An association between caesarean birth and increased risk of childhood obesity would provide an important rationale to avoid non-medically indicated caesarean section," write the authors.

Women who are considering a C-section as an alternative to natural birth in the absence of a medical indication should be warned of the potential health risks to their babies, including having a higher risk of obesity, researchers concluded.

In 2007, 32 percent of births in the U.S. were by C-section, which had gone up 20.7 percent in 1996, according to the latest statistics from the U.S. Centers for Disease Control, and an estimated 4 percent to 818 percent of C-section deliveries were done by “maternal request,” not because they are recommended by the doctor, researchers noted.