DNA may dictate your development, but you also wouldn’t be you without the unique mix of bacteria that make their home on your body. This week in the Proceedings of the National Academy of Sciences, researchers say that the very moment of your birth can decide for a lifetime what kind bacteria live in your body, and even whether you’ll be at a higher risk for conditions like asthma.

The uterus is a sterile environment. So, in the womb, babies don’t have any bacteria to call their own. It’s only once they enter the world that they begin to collect the microbes that will colonize their bodies and help shape their immunity [Scientific American].

How babies enter the world is the key, the team says. The studied surveyed the bacterial colonies of 10 mothers just before birth; four of those women gave birth traditionally and six did through cesarean section. When the scientists then checked up on the bacteria living in the newborns, they found that the difference in birth method decided what microbes the baby would get. Those born vaginally tended to pick up the bacteria from their mother’s vagina, while those born via C-section harbored bacterial colonies that tend to come from skin.

Dr Noah Fierer, one of the study leaders from the University of Colorado at Boulder, US, said: “In a sense the skin of newborn infants is like freshly tilled soil that is awaiting seeds for planting – in this case, bacterial communities. The microbial communities that cluster on newborns essentially act as their first inoculation.” He added: “In C-sections, the bacterial communities of infants could come from the first person to handle the baby, perhaps the father” [UK Press Association].

While C-sections have shot up in popularity and can be a life-saving procedure for the mother, this study suggests that the birth method can skew those “bacterial communities.” And the mix of skin bacteria that C-section babies pick up may not be as effective an inoculation.

Previous research suggests that babies born via C-section are more likely to develop allergies, asthma and other immune system–related troubles than are babies born the traditional way [Science News].

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