Oops, C-section does NOT impact the neonatal gut microbiome much at all!

Gut bacteria , gut flora, microbiome. Bacteria inside the small intestine, concept, representation. 3D illustration.

The more research we do, the LESS support there is for the claim that C-sections impact the neonatal microbiome in ways that are ultimately harmful to health.

Research on the impact of C-section on the neonatal microbiome has been plagued with major problems, the most important of which is the naturalistic fallacy: the belief that if something is a certain way in nature, that must be the best possible way. Since passage through the vagina was the only way to give birth for most of human history, there are many people who believe it must be the best way to give birth.

[pullquote align=”right” cite=”” link=”” color=”” class=”” size=””]The claim that C-sections alter the neonatal microbiome in ways that are harmful has never been proven and is likely to be nothing more than wishful thinking.[/pullquote]

Vaginal birth is obviously not the best way to maximize maternal and infant survival. C-sections save literally hundreds of thousands of mothers and babies each and every year. Research has shown that a minimal C-section rate of 19% is necessary to achieve low maternal and perinatal mortality. Those clinging to the naturalistic fallacy continue to search for something, anything, with which to demonize C-sections. The latest object of their affections is the neonatal microbiome. Many within the natural childbirth industry, and some within the medical profession, are claiming that C-section alters the neonatal microbiome in ways that are ultimately harmful to longterm health.

Last year I reported on the largest study to that date that looked at the impact of mode of delivery on the neonatal microbiome. Maturation of the infant microbiome community structure and function across multiple body sites and in relation to mode of delivery was published in Nature Medicine.

The authors found:

…[T]here was no discernable effect of the cesarean mode of delivery on the early microbiota beyond the immediate neonatal period (and never inclusive of that in the meconium or stool) …

Now comes a new literature review that further undermines the claim that C-sections alter the neonatal microbiome. A Critical Review of the Bacterial Baptism Hypothesis and the Impact of Cesarean Delivery on the Infant Microbiome was published just this week.

The authors take aim at the bacterial baptism hypothesis.

…[E]pidemiological studies have linked Cesarean delivery with increased rates of asthma, allergies, autoimmune disorders, and obesity. Mode of delivery has also been associated with differences in the infant microbiome. It has been suggested that these differences are attributable to the “bacterial baptism” of vaginal birth, which is bypassed in cesarean deliveries, and that the abnormal establishment of the early-life microbiome is the mediator of later-life adverse outcomes observed in cesarean delivered infants. This has led to the increasingly popular practice of “vaginal seeding”: the iatrogenic transfer of vaginal microbiota to the neonate to promote establishment of a “normal” infant microbiome.

The investigators who first proposed the bacterial baptism hypothesis noted differences in the microbiota of the nose and mouth between infants delivered by C-section vs vaginal birth.

Given that neonates were swabbed within seconds of delivery, and thus it would be coated with vaginal fluids, this result is hardly surprising. This does not necessarily demonstrate colonization, however.

What about the initial neonatal gut microbiome?

Numerous studies describing the bacterial microbiota of first pass meconium (the first fecal material, passed shortly after birth) support the notion that CSD and VD neonates do not differ in their bacterial microbiomes in the first few days following birth.

How about thereafter?

Although most studies report no differences in the microbiome of VD and CSD neonates in the first days of life, evidence is compelling that differences begin to develop shortly thereafter and persist for weeks or months.

And the differences almost entirely disappear when infants start eating solid food.

Do the temporary differences reflect mode of delivery or confounding factors? It is very likely they are the result of confounding factors such as:

1. Antibiotics:

All mothers delivering by CS are administered intrapartum antibiotic prophylaxis (IAP), as is routine for any type of surgery. In some countries, IAP is administered after the cord is clamped, minimizing direct antibiotic exposure of the neonate. In others, antibiotics are given prior to commencement of surgery… Mothers delivering vaginally are not routinely administered antibiotics, with the notable exception of those who are vaginally colonized with Group B Streptococcus (GBS). Overall, rates of intrapartum antibiotic use are low in vaginally delivering mothers.

2. Labor:

…[L]abor causes changes in levels of endocrine, inflammatory, and contractile factors. These changes might influence the maternal microbiome or the establishment of the neonatal microbiome. Additionally, labor is often accompanied by rupture of the fetal membranes, exposing the fetus to maternal vaginal bacteria…

3. Breastfeeding:

Source tracking studies have shown that 27% of an infant’s gut microbiota is vertically derived from its mother’s breast milk, while an additional 10% is sourced from the skin around the areola.

4. Maternal obesity:

Obesity and high-fat diets have repeatedly been correlated with aberrations to the gut microbiome in humans. Maternal obesity alters the maternal gut microbiome during pregnancy, and the milk microbiome during lactation …

The microbiome of obese mothers may have a harmful effect on weight gain in toddlers:

Mother-to-child transmission of obesogenic microbes continues to disrupt microbiome patterns into early childhood. Galley et al. found that the gut microbiomes of toddlers born to obese mothers of high socioeconomic status (SES) clustered away from those of toddlers born from lean high SES mothers. In particular, children born to obese mothers had differences in abundances of Faecalibacterium spp., Eubacterium spp., Oscillibacter spp., and Blautia spp., all of which have been correlated to diet and body weight in previous studies.

5. Gestational age and NICU exposure:

Rates of CS delivery increase with decreasing gestational age at delivery. Preterm infants differ from their full-term counterparts in terms of their gut microbiota, immune development, and health outcome…

The NICU environment is likely to influence the microbiome, so duration of residence and the environmental microbiome of the unit are likely to have a significant impact…

6. Inter-individual variation:

Studies that compare the microbiomes of infants born by CS or vaginal delivery must have sufficient power to account for variation in the maternal microbiome, as this is likely to exert a large influence on an infant’s microbiome through breastfeeding and physical contact. Large cohorts are thus required with the ability to control variables, such as home environment, presence of pets, and exposures to other microbiome-altering factors including hygiene and maternal/infant diet.

To date there have been no studies involving large cohorts.

In summary:

…[G]iven the numerous and significant confounding factors present in studies comparing the microbiota after CS and vaginal delivery, it is impossible to say with any certainty that it is the act of delivering vaginally itself which confers this optimal microbiota, or what species/genera of bacteria might be responsible. Differences in antibiotic administration, labor onset, maternal body weight and diet, gestational age, and breastfeeding frequency and duration undoubtedly contribute to differences observed between CSD and VD infants. Further, it is likely that differences between CSD and VD infants do not develop until several days after birth. Given recent evidence that infant microbiome colonization begins in utero, it may be that the importance of “bacterial baptism” of vaginal birth has been significantly over-estimated.

Although numerous studies have demonstrated an association between CS delivery and altered microbiome establishment, no studies have confirmed causality.

The authors recommend abandoning the practice of vaginal seeding:

Health practitioners should not bow to popular pressure to perform vaginal seeding in the absence of data on need, effectiveness, and appropriate protocols for ensuring safety.

The natural childbirth industry is not going to give up on demonizing C-sections any time soon, but women need to know that the claim that C-sections alter the neonatal microbiome in ways that are harmful has never been proven and is likely to be nothing more than wishful thinking.