A newborn's feet, days old, the exact age window this study begins tracking.
Janko Ferlič
Illustrative. Not a photograph of a participant in the study, which tracked gut bacteria through stool samples rather than photographing babies.
Your Baby's Gut Bacteria Come From Your Gut, Not the Birth Canal
Almost every seed your baby's microbiome pitch on the market points to two things: how you deliver and whether you breastfeed. A new Nature study followed 714 mothers and their babies for a full year, sequencing more than four thousand stool samples, and found that neither one is the main event.
- A Nature study of 714 mothers and infants, tracked from twelve weeks of pregnancy through the first year of life across 4,526 stool samples, found the mother's own gut, not the birth canal and not breast milk, is the main source of a newborn's gut bacteria.
- The transfer is proportional: the more of a species a mother carries, the more likely it lands in her baby, and the longer it persists once it does.
- Delivery mode and feeding mode still shaped the infant microbiome strongly across the first year, and exposures from before pregnancy, including smoking that had already stopped years earlier, showed up too.
- A preliminary finding: babies who later developed eczema had mothers with lower gut microbial diversity, though the researchers call this a first report that needs replication.
What the researchers actually did
The team, led by researchers at UMC Groningen, drew on the Dutch Lifelines NEXT birth cohort, tracking 714 mother and infant pairs from twelve weeks of pregnancy through the first year of the baby's life. They metagenomically sequenced 4,526 stool samples across that window and cross referenced the results against 474 clinical and exposure variables, plus deep sequencing of each mother's breast milk and vaginal microbiome. That scale is what let them separate three possible sources of a baby's gut bacteria, the mother's gut, the vaginal canal, and breast milk, and ask which one actually explains what shows up in the baby.
The mother's gut, not the birth canal, is the main source
The answer was not close. A large share of the bacterial strains found in an infant's gut were identical to strains already living in the mother's own gut, far more often than they matched anything from the vaginal microbiome or breast milk. Transmission from the vagina and from milk did happen, and it could matter when it did: the team traced Bifidobacterium breve, a strain often marketed as a key infant probiotic, moving specifically through the vaginal route into the infant gut. But as a rule, if you want to know where a newborn's gut bacteria came from, the mother's gut is the reservoir to look at first.
the mother's gut is the reservoir to look at first
The transfer also followed a clear pattern. The more abundant a bacterial species was in the mother's gut, the more likely it was to show up in her baby, and once it did, it tended to stick around longer. This is not a single handoff at birth. It looks more like the mother's own gut composition setting the odds for what her baby's gut ends up carrying, and for how long.
Delivery, feeding, and things that happened before pregnancy still matter
None of this makes birth mode or feeding mode irrelevant. Across the full first year, delivery mode and feeding mode remained dominant forces shaping the infant microbiome, and not just which microbes showed up. Delivery mode tracked with how well the infant's gut bacteria could break down the carbohydrates in human milk and formula. Feeding mode tracked with the microbiome's ability to produce compounds involved in gut to nervous system signaling. The mother's gut supplies the raw material; birth and feeding still shape what that material does.
The more surprising thread was how far back the influence reached. Exposures from before pregnancy showed up in the infant's microbiome months and years later, including maternal smoking that had already stopped long before conception, pre pregnancy BMI, infections during pregnancy, how long the membranes had ruptured before delivery, and whether there were siblings or pets already in the home.
One finding is preliminary but worth naming directly. Mothers whose babies later developed eczema had lower gut microbial diversity themselves, a pattern that held even after adjusting for maternal smoking history and family history of allergic disease. It is the first time this specific association has been reported, and the researchers are asking for it to be replicated before anyone treats it as settled.
Honest caveat
This is an observational study. It shows where an infant's gut bacteria come from; it does not prove that changing a mother's gut changes her baby's health, and no intervention was tested here. The eczema finding is the weakest and most attention grabbing part of the paper: it comes from a single cohort, it is a first report, the mechanism is unknown, and the researchers explicitly call for replication in a more diverse population before it means anything clinically. The cohort itself is Dutch and ethnically narrow, so how well this generalizes elsewhere is unproven. It is also worth knowing that Nestle helped fund and co develop the Lifelines NEXT cohort this study draws on. That does not make the findings wrong, but it does mean the commercial upside of a maternal first nutrition pitch belongs to a company that helped pay for the data behind it. To that company's credit, its own spokesperson said plainly, in the coverage of this study, that the findings do not show a maternal supplement improves infant health, and that proper trials would still be needed to claim that.
What this means for you
If you are pregnant or trying to conceive, the practical takeaway is not a new supplement to buy. It is that the everyday habits that build your own gut diversity, fiber, plant variety, fermented foods, less ultra processed food, appear to matter for a reason bigger than your own digestion. No maternal probiotic on the market has been shown in a real trial to improve an infant's health outcomes by way of this pathway, whatever the marketing implies. If you already had your baby by cesarean section or never breastfed, this study is also, in its own way, reassuring: the birth canal and breast milk were never the main event to begin with.
Primary sources
- Sinha T, Brushett S, Zhernakova A, et al. Maternal influences on infant gut microbiome and health. Nature, published August 12, 2026.
- UMCG Research. New research highlights the maternal impact on the infant gut microbiome and health. August 12, 2026.
- NutraIngredients (Cassandra Stern). Study: Maternal gut holds clues to infant health. September 15, 2026.
Common questions
Does this mean birth mode and breastfeeding do not matter for my baby's gut?
No. Delivery mode and feeding mode remained strong influences on the infant microbiome across the whole first year in this study, shaping things like how well a baby's gut bacteria can digest milk sugars. What changes is which route supplies the actual bacterial strains in the first place, and this study found that is mostly the mother's own gut, not the birth canal or breast milk.
Should I take a maternal probiotic to seed my baby's microbiome?
This study does not support that. It shows where an infant's gut bacteria come from, it does not test whether taking a supplement changes anything for the baby. Nestle, which helped fund the cohort behind this research, had its own spokesperson say directly that the findings do not show a maternal supplement improves infant health, and that real trials would still be needed to make that claim.
What is the eczema finding, and should I be worried about it?
It is preliminary. The researchers found that mothers whose babies later developed eczema had lower gut microbial diversity themselves, and the pattern held after adjusting for smoking history and family allergy history. But this comes from a single cohort, it is the first time this exact association has been reported, and the researchers say themselves it needs to be replicated in a different, more diverse group before it should change anyone's expectations.
Stay Curious.
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