There is a lot of talk online about the human microbiome (our gut bacteria)—and what impact exposure to mama's microbiome during birth can have on infants. Concerned parents may also have heard that early research showed that babies born via C-section didn't have the same "good" bacteria as those born vaginally.

A new large-scale study now gives us a better idea of what those differences actually are, and it's not what many assumed.

The results show that babies born via C-section do have different microbiomes than the vaginally delivered babies but the babies born vaginally and via C-section had about the same amount of Lactobacillus bacteria, with the vaginal canal. What the vaginal babies did have in greater abundance was Bacteroides bacteria, which means the difference is in exposure from the perineum, not the vaginal canal.

According to the researchers, vaginally born babies got most of their gut bacteria from their mother while c-section babies instead had more bacteria associated with hospital environments.

In a study published last month in the journal Nature, researchers compared the gut microbiomes of nearly 600 babies when they were 4, 7, and 21 days old, and again when they were about a year old. Known as the Baby Biome Study it was the largest one conducted on newborns' microbiome to date. The researchers found that vaginal seeding is unlikely to have the impact parents are aiming for. The American College of Obstetrics and Gynecologists advises against this procedure because we still lack proof it's safe and effective, and the researchers behind this study don't recommend it either.

"The results suggest that swabbing vaginal bacteria is unlikely to change the baby's gut microbiome," the study's authors state in an FAQ. The good news from this research is that a year after their birth, the differences in microbiomes disappeared between the two groups of babies.

The Royal College of Obstetricians and Gynecologists said pregnant women should not let this study deter them from following medical advice to have a C-section and researchers will continue to investigate how the microbiome impacts infant immune systems. "Further studies will help us understand the role of gut bacteria in early life and could help us develop therapeutics to create a healthy microbiome," Peter Brocklehurst, a professor at the University of Birmingham and the study's principal investigator, told the Telegraph.

[Correction, November 7, 2019: A previous version of this post suggested there's no big difference in baby's gut bacteria between C-sections and vaginal births. There is a difference, but they are largely evened out by 1 year old.]

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