June 15, 2023

expert reaction to study on vaginal microbiota seeding and neurodevelopment of babies born via c-section

A study published in Cell Host & Microbe looks at the effects of vaginal microbiota transfer on the neurodevelopment and microbiome of caesarean-born infants.

Dr Yan Shao, Staff Scientist at the Wellcome Sanger Institute, said:

“The concept of “vaginal seeding” to restore the perturbed microbiota of newborns delivered by C-section has been a topic of controversy among both scientific and medical communities since the initial pilot study in 2016 [1]. Current scientific evidences [1-6] overwhelmingly suggest that this procedure offers limited, if any, efficacy. In addition, the mother’s vaginal microbes rarely (and if they do, only transiently) colonize the infant’s gut. This is also consistent with the findings from us in the UK-based Baby Biome Study [7], and many other colleagues, that the bacteria depleted in the babies born by C-section (versus vaginally) are microbes originated from the mother’s gut rather than vagina, such as Bifidobacterium and Bacteroides.

“In this context, the positive efficacy reported in this randomised controlled trial study is surprising in several ways. Firstly, despite having a relatively small sample size (30 per group) and accounted for multiple confounding factors, the authors reported subtle but statistically significant differences associated with vaginal seeding in the gut microbiome and metabolome compositions of babies born via C-section. Counterintuitively, these minor differences only became apparent later in life but not closest to birth (time of intervention), towards the end of the sampling period (day 42).

“Furthermore, the authors attributed differences in the microbiome associated with vaginal seeding to accelerated gut microbiota maturation, based on an arbitrarily defined ‘trajectory’ from ‘stunted’ to ‘mature’ states around one month of age. However, considering that the infant gut microbiome would still be in a rapid developmental phase at this point and wouldn’t reach a stable maturation phase until after the first year of life, it’s inappropriate to interpret the observed differences as maturation. Instead, they more likely reflect the dynamic nature of temporal microbial community structures among infants at this age (day 42).

“Moreover, the only bacterial group that differentiated the seeded from non-seeded C-section babies was Lactobacillus, which is dominant in the mother’s vaginal fluid but contributed a very minor fraction (less than 5%) of the infant’s gut microbiome. This suggests that vaginal seeding might have only restored the transmission of mother’s vaginal bacteria, rather than the mother’s gut bacteria (Bifidobacterium and Bacteroides) that are expected to dominate in vaginally-born infants and be underrepresented in C-section babies. Without presenting and comparing to the bacteria that are depleted in C-section-born babies (relative to vaginally-born ones) in this cohort, it is challenging to establish the true effectiveness of vaginal seeding in restoring a ‘normal’ infant gut microbiome. In this respect, the study is also limited by the resolution of the 16S rRNA amplicon sequencing technique used for microbiome profiling. This method, in contrast to whole-genome metagenomic sequencing, precludes the authors from investigating the engraftment and maternal transmission of ‘seeded’ microbes at the species and strain-level, as done in recent studies.

“Perhaps the most novel and unexpected finding is that babies that had vaginal seeding also exhibited more ‘mature’ neurodevelopment at three and six months of age. Notably, this finding was statistically significant, despite the broad confidence interval and adjustments made for potential confounders. Intriguingly, the C-section babies who received vaginal seeding seemed to score even higher in neurodevelopment scores than the reference group of babies born via vaginal delivery. This poses the question: does vaginal seeding make a baby’s neurodevelopment more or less ‘normal’?  Without a clear understanding and definition of ‘normal’ microbiome development and neurodevelopment, and absent replication of these findings in larger cohorts with longer-term samplings, it’s too early to draw conclusions about the impact of vaginal swabbing (whether it can ‘normalize’ or ‘improve’ infant development as stated in this paper).”

[1] Partial restoration of the microbiota of cesarean-born infants via vaginal microbial transfer. Nat Med. 2016 Mar;22(3):250-3. doi: 10.1038/nm.4039

[2] Maternal Fecal Microbiota Transplantation in Cesarean-Born Infants Rapidly Restores Normal Gut Microbial Development: A Proof-of-Concept Study. Cell. 2020 Oct 15;183(2):324-334.e5. doi: 10.1016/j.cell.2020.08.047

[3] Oral administration of maternal vaginal microbes at birth to restore gut microbiome development in infants born by caesarean section: A pilot randomised placebo-controlled trial. EBioMedicine. 2021 Jul;69:103443. doi: 10.1016/j.ebiom.2021.103443

[4] Naturalization of the microbiota developmental trajectory of Cesarean-born neonates after vaginal seeding. Med. 2021 Aug 13;2(8):951-964.e5. doi: 10.1016/j.medj.2021.05.003

[5] Effects of vaginal seeding on gut microbiota, body mass index, and allergy risks in infants born through cesarean delivery: a randomized clinical trial. Am J Obstet Gynecol MFM. 2023 Jan;5(1):100793. doi: 10.1016/j.ajogmf.2022.100793

[6] Maternal Bacterial Engraftment in Multiple Body Sites of Cesarean Section Born Neonates after Vaginal Seeding-a Randomized Controlled Trial. mBio. 2023 Apr 19:e0049123. doi: 10.1128/mbio.00491-23

[7] Stunted microbiota and opportunistic pathogen colonization in caesarean-section birth. Nature. 2019 Oct;574(7776):117-121. doi: 10.1038/s41586-019-1560-1

[8] Temporal development of the gut microbiome in early childhood from the TEDDY study. Nature. 2018 Oct;562(7728):583-588. doi: 10.1038/s41586-018-0617-x

Prof Dorothy Bishop, Emeritus Professor of Developmental Neuropsychology, University of Oxford, said:

“This article presents pilot data from a study comparing outcomes of infants delivered by Cesarean section who were either exposed to maternal vaginal swabs, or control (saline) swabs with the goal of assessing the impact of transferring vaginal microbiota. The rationale is that prior studies have detected a higher risk of neurodevelopmental problems in infants delivered by Cesarean section, and it has been suggested this may reflect the fact that these infants are not exposed to their mother’s vaginal microbiota, as would happen with a normal vaginal delivery.

“My expertise is in neurodevelopment, not microbiology, and my comments are focused on the neurodevelopmental outcomes, as measured by a Chinese version of the Ages and Stages Questionnaire 3 (ASQ3). This is a widely-used parental report instrument, in which parents are asked six questions about development in each of five domains: communication, gross motor, fine motor, problem solving and personal-social development. More information is available here http://www.bestkc.com/wp-content/uploads/2017/05/3-year-ASQ-ENGLISH.pdf.  Responses are simply coded as YES (10 points), SOMETIMES (5 points) or NOT YET (0 points). There are different versions of ASQ3 suitable for different age bands. The 6 month ASQ3 was the primary outcome of this study. The ASQ was also administered at 3 months of age. (The authors confirmed they used the 4-month version of ASQ for the 3 month assessment).

“Although the focus of the article is on comparison of the two Cesarean groups, an additional comparison group with regular vaginal delivery (VD) was also included. This seems rather important, as the rationale of the study is that Cesarean delivery carries a risk that is not present with vaginal delivery, and which can be ameliorated with vaginal microbiota transfer (VMT). So the prediction would be that VD and VMT should have similar outcomes, and both be better than regular Cesarean cases (Con). The VD data on ASQ3 are reported in the Supplementary data, and these predictions seem broadly supported for the primary endpoint (6 month ASQ total).

“However, the clinical significance of these findings is not clear. ASQ scores are rather hard to interpret, as they vary with age, and are usually treated as screening measures, to identify children who score below a cutoff. As far as I could judge, the scores of the control Cesarean group seem pretty average – at least in relation to US data, and to such Chinese data as I could find. They do not look like a group with impaired neurodevelopment.

“The difference in scores for the two Cesarean groups seems driven predominantly by high scores for the VMT group, rather than low scores of the Con group, who do not appear to be impaired on the ASQ at either age. Another way of seeing if the Con group is impaired is to compare them to the non-Cesarean (VD) group. In statistical terms the Con group and VD group seem similar, although there is a trend for lower scores on some measures; note too that the sample size, especially for the VD group at 3 months, is too small to give a convincing test, and the study does not have adequate power to detect subtle differences.

“Overall, the authors note this is a pilot study, and using a clever design, the researchers have demonstrated that vaginal microbiota transfer is feasible and does not appear to have any associated risks. This study does not provide convincing evidence that the method protects against risk of neurodevelopmental disorder: the small sample and unusually high scores from the VMT group sound a note of caution. As the authors note, however, it would be worth doing a much larger study to establish whether an impact on neurodevelopmental outcomes can be confirmed, given that this is an acceptable and inexpensive intervention.”

‘Effects of vaginal microbiota transfer on the neurodevelopment and microbiome of cesarean-born infants: A triple-blind randomized controlled trial’ by Lepeng Zhou et al. was published in Cell Host & Microbe at 16:00 UK time on Thursday 15^th June.

DOI: http://dx.doi.org/10.1016/j.chom.2023.05.022

Declared interests

Dr Yan Shao works on the early-life gut microbiomes in UK birth cohorts, including the Baby Biome Study and the Children Growing Up in Liverpool (C-GULL) Study. He declares no conflicts of interests.

Prof Dorothy Bishop: “I declare I have no conflict of interest.”