Birth exposes babies to maternal gut bacteria that help build early microbiomes

The very first thing many babies encounter when they’re born is their mother’s poop. If they’re born vaginally, head down, with the face against their mother’s rectum, the pressure can expel fecal material just as they are born. This early exposure is not only normal but likely to be beneficial.

Microbes from their mother acquired during birth, and especially from the mother’s poop, help shape a baby’s developing gut and immune system, setting the stage for lifelong health.

Birth is a microbial event

Birth is the moment when babies are exposed to and colonized by live bacteria. As they move through the birth canal, they are coated in the mother’s vaginal bacteria.

Historically, we believed the dominant bacterium in the birth canal—Lactobacillus—was the first and most important microbe to seed the baby’s gut. Lactobacillus is easy to grow in the lab, extensively studied and widely recognized as a “good” bacterium.

But although babies encounter Lactobacillus during birth, it does not remain in their gut for long.

Instead, the microbes that form the foundation of a newborn’s gut microbiome come mainly from the mother’s gut, not her vagina.

This raises an obvious question: How do babies acquire their mother’s gut bacteria?

The answer lies in the mechanics of birth.

It’s a tight squeeze

The ideal delivery position in a vaginal birth is known as “occiput anterior.” This is when the baby’s head is face down, facing the mother’s back, with their chin tucked down. So the baby’s head presses against the mother’s rectum as it descends, expelling fecal material just as the baby is born.

The truth is that the first thing a baby sees on entering this world is their mother’s intestinal contents. Far from being an accident, this appears to be an evolved mechanism that ensures the baby receives the microbes best suited to colonize their gut.

But we want to be clear. Deliberately smearing poop on your newborn’s face is dangerous and can lead to a life-threatening infection.

That’s because the amount and source of bacteria, and the way these are delivered, won’t match what happens naturally. Also, newborns’ immature immune systems make them especially vulnerable to infection.

Then there’s breast milk

Nature has refined this process even further through breast milk. This is because breast milk not only has all the nutrients a baby needs but also contains molecules known as “human milk oligosaccharides.” These are the third most abundant solid in breast milk, yet babies cannot digest them.

So what are they doing there?

These oligosaccharides are food for specific beneficial bacteria, especially Bifidobacterium. This is one of the key microbes transferred from mother to baby at birth.

These bacteria use these oligosaccharides to grow, protect the baby’s gut from harmful microbes, support immune development and help the immune system learn when to respond and when to stay calm.

But this system can be disrupted

This coordinated system—birth exposure, maternal gut microbes and breast milk—has evolved to optimize babies’ survival.

But medical practices can disrupt it. Antibiotics, while lifesaving, alter the mother’s microbiome. Cesarean delivery bypasses exposure to the key vaginal and intestinal microbes.

Babies born by cesarean or exposed to antibiotics during pregnancy have different early microbiomes and may face increased risks of certain childhood conditions, such as asthma and atopic dermatitis.

These medical interventions are essential when needed and should never be avoided out of fear of disrupting the microbiome. The health and safety of mother and baby always come first.

However, understanding how these interventions affect how babies acquire beneficial bacteria helps us identify ways to support babies when this natural exposure is interrupted.

How can we use this?

One area where this knowledge has had a profound impact is the care of babies born before 33 weeks.

These preterm babies undergo numerous medical procedures and often miss out on normal microbial exposure. They are also at high risk of serious conditions such as necrotizing enterocolitis, when parts of the bowel die.

Giving these babies probiotics containing Bifidobacterium significantly reduces the incidence and severity of necrotizing enterocolitis.

This makes sense. Bifidobacterium is a key early-life microbe, perfectly adapted to thrive on human milk oligosaccharides in breast milk, and may even be transferred through breast milk itself.

In a nutshell

In essence, birth is not just a physical event; it is a microbial one. Nature has orchestrated a system where the mother passes on the microbes and nutrients her baby needs to thrive.

While medicine today sometimes alters this process, our growing understanding of the microbiome allows us to support babies in new and effective ways.

Who’s behind this story?

Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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This article is republished from The Conversation under a Creative Commons license. Read the original article.