Swabbing caesarean-section babies with their mum’s vaginal fluids to encourage the development of a microbiome seems to be safe — and might improve brain development, finds the biggest-yet study of the controversial practice1.
The randomized and blinded trial of 76 infants and mothers found that babies who had been swabbed with vaginal fluids from their mothers achieved a slightly higher number of neurodevelopmental milestones by three and six months than did c-section babies who did not receive the treatment. But it is not clear whether these differences will be meaningful or lasting, say scientists.
Could baby’s first bacteria take root before birth?
“This is not going to make a difference between going to Harvard or not,” says Jose Clemente, a microbiologist at Icahn School of Medicine at Mount Sinai in New York and a co-author of the study, which was conducted at a hospital in China. “It’s up to future studies to determine exactly what the mechanism is and how can we maximize the benefit.”
Mothers who deliver their babies naturally provide them with the beginnings of a microbiome — the body’s collection of microbes in the gut, on skin and elsewhere. Vaginal seeding involves collecting a swab of vaginal fluids from the mother and smearing them soon after birth on the skin of a baby born by c-section, with the aim of replicating this first exposure.
Studies have identified differences in the microbiomes of c-section babies and those delivered vaginally2. C-section babies had higher levels of opportunistic bacteria — those that circulate in hospital — in their guts in the days after birth, and, months later, tended to lack common gut microbes that would support immune function.
In 2016, a team co-led by Clemente published a trial of vaginal seeding: four infants who underwent the procedure successfully acquired microbes from their mother’s swabs and had microbiomes that resembled those of children delivered vaginally3.
Although the study did not look at the long-term effects of vaginal seeding, the findings led to an avalanche of interest in the procedure, says Clemente. In 2017, the American College of Obstetricians and Gynecologists expressed concern that vaginal seeding could transmit pathogens and recommended that the procedure be conducted only as part of clinical trials4.
To better understand the safety of vaginal seeding and determine its potential benefits, a team led by clinical scientist Yan He at Southern Medical University in Guangzhou, China, recruited women who were due to deliver by c-section and randomly selected them to have their newborns exposed to either vaginal swabs or sterile saline.
Women were first screened for numerous infectious diseases that could threaten the health of the babies — including COVID-19, because the trial was conducted at the height of the pandemic. Babies who received the vaginal swabs developed no serious health problems, and had similar rates of non-serious complications, including mild skin conditions and fever, as the babies swabbed with saline.
When the babies were three then six months old, the researchers asked their parents to fill out a checklist of neurodevelopmental milestones in communication, movement, problem-solving and social and personal skills, such as reaching out for toys and smiling at their reflections in a mirror. The infants who received the vaginal swabs tended to reach slightly more of their milestones than did babies swabbed with saline.
“It’s very exciting and promising,” says Alexander Khoruts, a physician-scientist at the University of Minnesota in Minneapolis. But it’s not clear whether the neurodevelopmental differences will turn out to be meaningful, because the brain changes so much as children develop. “Early delay by a couple months may not translate into anything meaningful at 18 years.”
Lars Engstrand, a clinical microbiologist at the Karolinska Institute in Stockholm, is reassured by the safety of vaginal seeding, but only if the mothers are screened for pathogens. “That is a really important message to clinical researchers in this field.”
He agrees that his team’s results should be taken with a grain of salt. “I don’t think our data now has real clinical significance to say ‘yes, you should do this intervention’,” he says. Rather, he says it gives him the confidence to embark on a larger and longer-term study that will ask whether vaginal seeding affects rates of neurodevelopmental conditions such as attention deficit hyperactivity disorder and autism spectrum disorder, which population-health studies suggest are more common in people born through c-section.
Other clinical trials are also trying to pinpoint potential benefits of vaginal seeding. Engstrand and his colleagues are measuring its effect on a skin condition called atopic dermatitis in around 300 participants. Clemente is also part of a US study that is looking at allergies and asthma, and a team led by Maria Gloria Dominguez-Bello, a microbial ecologist at Rutgers University in New Brunswick, New Jersery, is looking at its effects on obesity.
Vaginal swabs are not the only way to alter the microbiomes of c-section babies: a 2020 study found that giving newborns diluted faecal samples from their mothers led to gut microbiomes that resembled those of children delivered vaginally5. Khoruts says that such observations should stimulate researchers to identify the specific microbes transferred during birth. “That’s what the next steps have to be.”