We used to think fetuses had no bacteria in their gastrointestinal tract (the gut) until they began to accumulate microbes (bacteria, viruses, and other bugs) on their way through their mother’s vagina.
But this theory was challenged when bacteria were found in meconium (the first bowel movement) of premature babies. This, of course, traveled through the gut, accumulating microbes along the way.
What’s clear is that newborns have little (if any) diversity in their gut microbiota—the collection of bacteria that accumulate in the gut. The diversity increases as they’re exposed to different environments.
The particular makeup of a newborn’s gut microbes is important, as it has been shown to affect their risk of developing certain diseases in childhood and adulthood.
Vaginal or Cesarean Birth?
The mode of delivery has a big impact on an infant’s microbiota. During natural delivery, the direct contact with the mother’s vaginal and intestinal flora help shape the colonization of the newborn’s gut bacteria. Newborns delivered via cesarean section don’t have this direct contact.
One study found that newborns born vaginally were colonized by Lactobacillus whereas cesarean delivery newborns were colonized by a mixture of bacteria typically found on the skin and in hospitals, such as Staphylococcus and Acinetobacter.
These early differences tend to be sustained. One study showed the distinct gut flora of infants born by cesarean delivery persisted at six months after birth. Fecal Clostridia numbers in 7-year-old children born vaginally were found to be significantly higher than in children of the same age born via cesarean.
But we still don’t know how this impacts children’s health and the risk of disease.
The Developing Immune System
We’re beginning to realize gut bacteria play an important role in the growth of infants’ immune systems. One way this could occur is by altering the development of the white blood cells that provide a first-line defense against invading microbes (the bugs that make us sick).
Research shows mice born in a germ-free environment have fewer of these white blood cells compared to healthy mice with a normal gut bacteria population. Such mice are also more prone to bacterial infections.
Allergic diseases such as asthma and hay fever appear more often in infants born via cesarean delivery than in those born vaginally.
It’s important to note that not all cesarean deliveries are the same. Some women have cesarean sections after a long labor during which their waters have broken. In this case, the infant would be exposed to quite a different microbial environment than a planned cesarean section carried out before her waters have broken.
Breast- or Bottle-Fed?
Breast-fed newborns have a distinctly different gut microbiome to other newborns. They have higher proportions of the beneficial bacterial species Bifidiobacterium than formula-fed infants. This is likely due to breast milk containing a type of prebiotic that facilitates the growth of bacteria such as Bifidobacterium.
Interestingly, when breast-fed infants are supplemented with formula feeds, their gut microbiota resembles infants who are exclusively formula-fed.
What does this mean for infants’ risk of developing disease?
A study of U.S. infants has shown that breast-fed babies had a gut microbiome that was richer in genes associated with “virulence” (in this context, the ability to fight off antibiotics and toxic compounds). These same babies also developed changes in the genes of their gut immune system that allowed them to better fight off infections.
This suggests that breast milk can promote a healthy biological crosstalk between the baby’s immune system and the gut microbiome.
Breast-feeding has been shown to reduce the development of necrotizing enterocolitis (when parts of the bowel die off) in newborns, as well as allergic and autoimmune diseases in childhood, including celiac disease, Type 1 diabetes, and asthma.
What if your child is born via a cesarean section and can’t be breast-fed?
Don’t worry, not all such infants will be at greater risk of developing autoimmune and allergic diseases. A whole host of environmental and genetic factors play a role in determining individual risk.
Vaginal seeding has recently been proposed as one way infants born by cesarean section might gain some of the protective effects of environmental exposure for their gut microbiota.
A proof-of-concept study in 18 infants published earlier this year showed that transferring vaginal fluid to newborns (via a swab across their mouth, nose, and face) shortly after delivery by cesarean section can result in microbiome profiles resembling that of infants delivered vaginally.
It is unknown, though, whether colonization in this way is partially or fully equivalent to the microbial transfer during labor. We also don’t know whether later health outcomes in these infants are affected by the practice.
Some expert clinicians warn against vaginal seeding because of the potential for unrecognized infections to be transmitted from mother to newborn. There is a risk, for example, of transmitting undiagnosed group B Streptococcus to newborns, given 12 to 15 percent of women have this organism in their vaginal fluid.
Better Ways to Catch Up
For now, it’s more sensible to focus on the practices that have been shown to promote microbiome development in newborns delivered by cesarean. These include delaying the first bath until after 12 hours, placing the newborn on the mother’s skin during the first few minutes after delivery, and breastfeeding in the operating room, if permissible.
There has been much effort made in simulating the composition of human milk through adding live gut bacteria (probiotics) as well as non-digestible fibers (prebiotics) to formula. This is thought to assist microbial colonization and immune responses in formula-fed infants in a similar way to breast feeding.
But hard data is lacking on whether this approach can lead to a real-life benefit, particularly when it comes to reducing the risk of allergic disorders.
Fortunately, a large clinical research project in New Zealand—the Probiotics in Pregnancy Study—will soon be able to answer this question.
Four hundred pregnant women expecting infants at high risk of allergic disease were given either the probiotic Lactobacillus rhamnosus or a placebo, from 14 to 16 weeks into their pregnancy until they gave birth or, if they breast-fed, for six months after.
The researchers will then check if the infant develops allergies such as eczema. The results may be instrumental in helping to shape health policy.