In the on going effort to demonize cesareans, the natural childbirth community seized on a tiny study and spun it into the claim that C-sections change the infant microbiome. A new, far larger study, shows that it does not.
As the Houston Chronicle explains:
[pullquote align=”right” cite=”” link=”” color=”” class=”” size=””]The infant gut microbiome is probably determined in utero.[/pullquote]
In a new study of 160 pregnant mothers and their babies published Monday, Aagaard and a team of Baylor researchers found that any differences in the microbiome of babies born via C-section are not the result of the birthing process …
“We do not see a lasting association between cesarean delivery and a distinct microbiome community or its function in infants beyond the neonatal period,” Aagaard said…
Where did the erroneous claim come from. It started with a 2010 paper by Dominguez-Bello that involved only 9 mothers (10 babies) that looked at swabs of the baby’s skin, mouth and nose and meconium within the first 24 hours after birth. You don’t have to be a scientist to understand that an observational study involving 10 babies for only 24 hours cannot be extrapolated to the population at large.
But Dominguez-Bello did just that anyway. Instead of undertaking a larger study to replicate her findings, she moved on the studying vaginal “seeding,” wiping C-section babies with their mother’s vaginal secretions. That study involved 18 mothers, 11 of whom delivered by C-section; only 4 babies were swabbed. The authors collected 6 sets of samples (oral, anal, skin) over 30 days. The authors claimed to find “partial restoration” of the infant microbiome.
In contrast, the new study by Aagaard et al. is far more robust. This study differed from the original studies in critical ways:
- More participants: At 81 participants, this study is 9X larger than the original study.
- More sample sites: Skin, mouth, nose and stool.
- Longer follow up: The sampling was continued to 6 weeks of age.
What did they find?
1. The infant microbiome is not homogenous. As with the adult microbiome, it differs substantially depending on where in or on the body you take samples.
We found that the neonatal microbiota and its associated functional pathways were relatively homogeneous across all body sites at delivery, with the notable exception of the neonatal meconium. However, by 6 weeks after delivery, the infant microbiota structure and function had substantially expanded and diversified, with the body site serving as the primary determinant of the composition of the bacterial community and its functional capacity.
2. Although there were some differences in microbiota between cesarean and vaginally born infants immediately after birth, these differences were restricted to the mouth, nose and skin. Gut bacteria did not differ based on mode of delivery.
Although minor variations in the neonatal (immediately at birth) microbiota community structure were associated with the cesarean mode of delivery in some body sites (oral gingiva, nares and skin; R2 = 0.038), this was not true for neonatal stool (meconium; Mann–Whitney P > 0.05), and there was no observable difference in community function regardless of delivery mode.
3. The infant microbiome changes rapidly in the first 6 weeks. By 6 weeks of age there was no detectable difference between babies born by C-section and those born vaginally.
For infants at 6 weeks of age, the microbiota structure and function had expanded and diversified with demonstrable body site specificity (P < 0.001, R2 = 0.189) but without discernable differences in community structure or function between infants delivered vaginally or by cesarean surgery (P = 0.057, R2 = 0.007).
4. The environment in the uterus is probably not sterile as previously thought. Thus the composition of the infant microbiome may be determined in utero, not at the time of birth.
unlike the skin, oral cavity or nares microbiota, the neonatal gut microbiota at the time of delivery did not significantly vary by mode of delivery. The content of the first meconium is hypothesized to reflect the in utero environment (in which the infant is swallowing amniotic fluid continuously from mid to late gestation), and thus we speculate that these microbes were similarly transmitted from the mother to the fetus during gestation, suggesting that seeding of the early microbiota may occur earlier than was previously thought.
The authors conclude:
In summary, we undertook the largest study to date … to analyze both the composition and function of the neonatal and infant microbiota with paired maternal–infant subjects across multiple body sites. We observed that by 6 weeks of age, the microbial community structure and function had significantly expanded and diversified. We further demonstrated that there 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) …
In other words, C-sections do not change the infant gut microbiome.
