Babies Inherit a Third of Mom’s Gut Bacteria, Study Finds

Every baby is born already carrying a signature of its mother's microbiome, and a new study puts an actual number on how much transfers in the first months of life. Researchers tracked 68 mother-infant pairs using genome-level DNA sequencing and found that infants share more than a third of their gut bacterial species with their mothers, but how a baby is born changes that picture substantially. The findings, published in Cell Reports, add strain-level detail to a question with real stakes: the earliest gut bacteria a baby picks up help set up immune and metabolic systems that keep working for years afterward.

Infants Share More Than a Third of Their Gut Species With Mom

At 2 to 4 months old, infants share an average of 35.1% of the bacterial species in their gut with their mother's combined gut and vaginal microbiome, according to the analysis led by epidemiologist Noel T. Mueller and colleagues at Johns Hopkins, the University of Colorado, and Michigan State University. The team used shotgun metagenomic sequencing (a method that reads nearly all the DNA in a stool or vaginal sample, instead of just one marker gene) on samples from the 68 mothers and their infants.

Most of that shared material traces back to one source: the maternal gut, not the vagina. On average, 33.7% of an infant's gut species overlapped with their mother's gut microbiome, compared with just 1.8% overlapping with her vaginal microbiome, a large and statistically significant gap (p < 0.001). Despite the vaginal canal being the more commonly assumed route of transmission during birth, the gut appears to be doing most of the seeding.

C-Section Delivery Cuts Down How Much Bacteria Passes to Baby

Babies born vaginally picked up noticeably more of their mother's bacteria than babies delivered by cesarean section (C-section), the study found. Vaginally delivered infants shared an average of 38.0% of species with their mothers, compared with 24.7% for infants born via C-section (p = 0.003). The gap held up at the strain level too: among mother-infant pairs with detectable sharing of Bacteroides (a genus of gut bacteria that breaks down a wide range of dietary fibers), the odds of a matching strain passing to the infant were roughly 8 times higher in vaginal deliveries than C-section deliveries (95% CI: 1.06 to 65.1, p = 0.044).

Bacteroides and Bifidobacterium, the two genera the study focused on, matter because prior research has linked delayed colonization by these microbes after C-section to differences in early immune training, with implications researchers have proposed for later conditions like eczema, asthma, and other allergic disease. This new study did not test those health outcomes directly. What it adds is a strain-resolved picture of exactly how much maternal bacterial inheritance is reduced when birth happens surgically, building on other recent strain-tracking work showing C-section disrupts mother-to-infant strain inheritance.

68 Mother-Infant Pairs Tracked With Genome-Level Sequencing

The researchers enrolled 68 mother-infant pairs through the National Institutes of Health's Environmental influences on Infant Health Outcomes (ECHO) program, collecting a fecal and vaginal sample from each mother during her third trimester of pregnancy, plus at least one infant stool sample at 2 to 4 months old. Of the mothers, 53 delivered vaginally and 15 by C-section. A subset of 42 infants provided repeat stool samples across their first year, with some sampled as late as 14 months.

To go beyond simple species counts, the team used a strain-tracking tool called StrainPhlAn, which can tell whether the exact bacterial strain in a baby's gut genetically matches the strain in the mother, rather than just sharing the same species name (two babies could both carry Bacteroides uniformis that are genetically distinct strains from entirely different sources). That distinction matters because the health effects of gut bacteria are often strain-specific rather than species-wide, a point a recent meta-analysis of mother-to-infant Bifidobacterium transmission has also emphasized across the broader field.

What This Might Mean If You're Managing Your Baby's Gut Health

A C-section birth does not automatically mean lasting harm to a baby's gut microbiome, and the study points to at least one factor a parent can influence either way: breastfeeding. Among exclusively breastfed infants, the ratio of Bifidobacterium infantis (a species specialized for digesting sugars unique to human breast milk) to the related but less specialized Bifidobacterium longum was significantly higher than in infants who were not exclusively breastfed (p = 0.031). That breastfeeding boost to B. infantis was strongest specifically among vaginally delivered infants (p = 0.009), and was not statistically significant among C-section infants (p = 0.757).

In plain terms, breastfeeding appears to reward B. infantis regardless of delivery mode, but its effect combines most strongly with vaginal delivery. That is a reasonable thing to raise with a pediatrician if gut health is a concern, though the researchers stop short of recommending any specific intervention, such as a B. infantis probiotic supplement, as an established fix. Other research on B. infantis as an infant gut colonizer has explored supplementation directly, but that is a separate question from what this study measured, which was natural transmission from mother to baby.

The Study's Small, Single-Cohort Sample Is a Real Limitation

A sample of 68 mother-infant pairs is modest for a field that increasingly relies on cohorts in the thousands, and the researchers themselves note that several patterns, like changes in vaginal-microbe sharing over an infant's first year, did not reach statistical significance and need replication in larger groups. The mothers in this cohort were also demographically narrow: 90% self-identified as white and 93% as non-Hispanic, so the findings may not generalize to more diverse populations.

The team also did not collect breast milk microbiome samples, so they cannot fully rule out breast milk, rather than the vagina, as the true source of the rare Bifidobacterium strains that a handful of mothers appeared to share with their infants through the vaginal route. Vaginal samples were also harder to sequence precisely than stool samples, because they carry more contaminating human DNA, a technical limitation the authors flag directly.

Larger, More Diverse Studies Are the Next Step

The authors call for larger, more diverse longitudinal cohorts that sample more maternal body sites (including breast milk) at matched sequencing depth. There is no announced follow-up trial yet, but the ECHO program's ongoing mother-infant sequencing effort sets up a natural next round of data to test whether these transmission patterns actually predict childhood health outcomes, rather than just microbial overlap on its own.

Common Questions

Does a C-section mean my baby has a "worse" gut microbiome? Not necessarily worse, but measurably different. C-section infants in this study shared fewer bacterial species and strains with their mothers overall, but the study did not test whether that difference translates into worse health outcomes.

Can breastfeeding make up for a C-section? This study suggests breastfeeding independently boosts Bifidobacterium infantis regardless of birth mode, though the effect was strongest and only reached statistical significance in vaginally delivered infants.

What is Bifidobacterium infantis and why does it come up so much? It is a bacterial species specially equipped to digest sugars found only in human breast milk (human milk oligosaccharides), and it is considered one of the more beneficial early gut colonizers in infants.

Was the vaginal microbiome unimportant for infant gut health in this study? It contributed far less overall, averaging about 1.8% of shared species compared with 33.7% from the maternal gut, but it mattered specifically for a small subset of mothers who carried Bifidobacterium in their vaginal microbiome and passed those exact strains to their infants.

Key Takeaways

• Infants share about 35% of their gut bacterial species with their mother in the first few months of life, mostly inherited from her gut rather than her vaginal microbiome.
• Vaginal delivery passes on meaningfully more maternal bacteria, at both the species and strain level, than C-section delivery.
• Breastfeeding independently raises Bifidobacterium infantis, a beneficial breast-milk-digesting microbe, with the clearest boost seen in vaginally delivered infants.
• The study is based on 68 mother-infant pairs from one mostly white, non-Hispanic US cohort, so larger and more diverse studies are needed before drawing firm conclusions about health outcomes.

References

1. Noel T. Mueller, Shaoming Xiao, Tiange Liu, et al. (2026). Mother-infant sharing of gut and vaginal microbes at the species and strain level. Cell Reports. https://doi.org/10.1016/j.celrep.2026.117920
2. Delayed Establishment of Gut Microbiota in Infants Delivered by Cesarean Section. NIH. https://pmc.ncbi.nlm.nih.gov/articles/PMC7516058/
3. Ru Yang, Yinan Wang, Zhiye Ying, et al. (2024). Inspecting mother-to-infant microbiota transmission: disturbance of strain inheritance by cesarian section. Frontiers in Microbiology. https://doi.org/10.3389/fmicb.2024.1292377
4. Eduard Flores Ventura, María Esteban-Torres, Miguel Gueimonde, et al. (2025). Mother-to-infant vertical transmission in early life: a systematic review and proportional meta-analysis of Bifidobacterium strain transmissibility. npj Biofilms and Microbiomes. https://doi.org/10.1038/s41522-025-00720-y