To do that, Costello's co-lead researcher Maria Dominguez-Bello of the University of Puerto Rico took samples of bacteria from mothers and babies immediately before and after birth. Women who gave birth vaginally transferred the unique mix, their unique "community" of vaginal bacterial to the newborn baby.

"And that was in direct contrast with the c-section babies, who harbored bacterial communities that resembled human skin, but not their own mother's human skin any more than any of the other mothers in the study."

The researchers suspect that the newborn c-section babies might be picking up their generic bacteria mix from the air - one way bacteria gets on our skin.

Exactly how the differences in bacteria might make vaginally born babies less prone to allergies and asthma isn't yet clear, but it does point to a difference that researchers can look at going forward. Costello says she expects more studies as DNA analysis gets cheaper and faster.

So, does Costello's study support the notion that vaginal birth is somehow "better" than a cesarean? Speaking via Skype, she says no.

"I think further studies will be necessary to show whether the differences we've demonstrated depending on delivery mode of babies when they first emerge into the world actually carry over time, and whether those differences might contribute to health outcomes."

Elizabeth Costello's paper is published online by the