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abies born by C-section miss out on all the healthy bacteria in the birth canal — but a simple swab could one day help restore those missing microbes, possibly reducing the infants’ risks of developing asthma, allergies, and other conditions.

The new study, published Monday in Nature Medicine, used a very small sample, so experts caution that the results are preliminary. They also caution new parents not to try the procedure on their own, as it could be dangerous for the newborn if their mothers carry certain viruses.

But the research suggests that swabbing babies born by Cesarean section with microbes collected from their mothers could help them develop healthy microbiomes — the colonies of trillions of microorganisms that thrive in the human gut and mouth, and also on our skin.

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That’s an exciting prospect for doctors and parents who’ve worried about the impact of a C-section on a child’s health.

Researchers swabbed four babies born by scheduled C-section all over with gauze that had been exposed to a mother’s vaginal fluids for about an hour. The swabbing took place within two minutes of the babies’ births. The study compared those infants with 11 babies who were born by scheduled C-section but not swabbed and with seven babies born vaginally.

Thirty days after they were born, the babies that were swabbed with their mothers’ microbes had skin and oral microbiomes similar to those born vaginally. By contrast, vaginal bacteria were much less present in the babies born by C-section who didn’t get swabbed.

The finding is a step forward in understanding how the early microbiome can affect a child’s health. Previous studies have suggested a correlation between delivery by C-section and an increased risk of asthma, allergies, obesity, and other immune conditions and metabolic disorders, though the reason for that association is not clear. The C-section rate has risen rapidly in the United States; one in three babies is now delivered surgically.

The new study is “very nice, beautiful, preliminary research,” said Dr. Josef Neu, a neonatologist and gastric health expert at the University of Florida who was not affiliated with the study.

But the results are just that — a proof of concept, not a prescription to start swabbing babies.

“These are preliminary results of a few infants to show the principle,” said study author and microbiome researcher Maria Dominguez-Bello of New York University. The next study is expected to take place in about 70 infants and will follow up with those infants after one year.

Neu said he’s aware of new parents who have asked obstetricians to swab babies with vaginal microbes because they’ve heard about research in the field. But Neu said the lack of robust evidence on the true benefits and risks of the procedure should give doctors and parents pause.

“I’m concerned that this will take off and parents are going to be asking their obstetricians or midwives to do this,” Neu said, “and we’re going to start seeing some babies with disastrous results because of this.”

Neu said he’s nervous that doctors will take vaginal microbes from mothers who have conditions they’re not aware of, like herpes, and then swab a newborn’s mouth with that virus, potentially spreading it. Babies born vaginally to moms with the virus face the same risk, but C-sections are normally performed in a way that minimizes the chances of that happening.

“There are a lot of issues that need to be dissected before we can say that this is a technique that should be used routinely,” Neu said.

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