Those of us who have a child who was born by Cesarean (C-section) have always taken pride in the fact that our babies’ heads are much rounder than those of babies born naturally. But those perfectly formed heads may come with a cost. Two new studies have found that C-section babies are more likely to be overweight as teens and to suffer from allergies than babies born vaginally.

In the first study, Dr. Jan Blustein and her team at the New York University School of Medicine, tracked 10,000 children from birth through age 10. She found that C-section babies were 83 percent more likely to be overweight or obese compared to the naturally-born children. This was after factoring in all other relevant information including the mother’s weight and whether she breastfed and for how long. As early as six weeks of age, the C-section babies were consistently heavier than naturally-born infants at almost all the check-ups monitored by the research team. Why is this happening? Blustein isn’t completely sure, but she speculates that 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. She published her findings in the journal International Journal of Obesity.

The second study, this one conducted by Christine Cole Johnson, Ph.D., chair of Henry Ford Department of Health Sciences, found that babies born by C-section have a higher risk of developing allergies than other babies. Again, the culprit may be the friendly bacteria that vaginally born babies pick up as they move through the birth canal. C-section babies also seem to have other microorganisms in their gut that make them more vulnerable to the antibodies that produce allergic reactions. It’s possible that those microorganisms would be squeezed out of a baby’s system during a normal birth. “This further advances the hygiene hypothesis that early childhood exposure to microorganisms affects the immune system’s development and onset of allergies. We believe a baby’s exposure to bacteria in the birth canal is a major influencer on their immune system,” said Dr. Johnson, the study’s lead author.