The placenta’s microbiome may hold surprising secrets (Image: Burger/Phanie/REX)

Babies in the womb are not as sheltered from the outside world as you might think. The placenta harbours a unique ecosystem of bacteria which may have a surprising origin – the mother’s mouth.

Disturbances of the placenta’s bacterial community may explain why some women give birth prematurely, and could also be one of the ways that a woman’s diet affects her offspring’s gut bacteria, and as a result the child’s disease risk. “Different nutrients [in the mother’s diet] are a huge determinant of which microbes take up residence in the placenta,” says Kjersti Aagaard of Texas Children’s Hospital and Baylor College of Medicine in Houston, who led the study.

In the past decade there has been growing awareness of the important role of the human microbiome – all the bacteria, viruses and fungi that live on and in our bodies. Disturbances to the gut microbiome have been linked with conditions ranging from obesity to autism.


Until recently it was generally thought that babies are born with a sterile gut and that they pick up microbes on their journey through their mother’s vagina which migrate to colonise the gut. Additional microbes are collected from their environment over the first few years of life. This theory was challenged when bacteria were found in the meconium, a baby’s first stool passed within hours of their birth.

We now have a clue to where these bugs are coming from. Aagaard and her colleagues genetically sequenced the bacteria of the placenta, the organ that transfers nutrients and oxygen to the fetus from the mother’s blood. They took samples from the placentas of 320 women after they had given birth, taking tissue from inside the placenta to avoid any contamination by vaginal bacteria.

Jaw-dropping origins

The team found a broad range of bacteria present, including those necessary for metabolising some of the vitamins and nutrients needed by the fetus. The first surprise was that the bacterial species were most similar to those normally found in the adult mouth, as opposed to the vagina or gut. “The placenta has its own ecology and these were not the bacteria we were expecting,” says James Kinross, a colon surgeon at Imperial College London who researches gut bacteria and was not involved in the new work. “Most people would have expected it to be a vaginal flora,” he says, because of its close proximity.

The fact that it was most similar to the bacterial community found in our mouths suggests that these bacteria are somehow finding their way through the bloodstream to the placenta. Having got that far, they could then reach the baby either by crossing into the baby’s blood vessels within the placenta or by passing into amniotic fluid, which is swallowed by the baby, suggests Aagaard.

The team also found that some bacterial species were more common and others less common in the women who had given birth prematurely – before 37 weeks of pregnancy – than the typical bacterial profile of the women who went to full term. Intriguingly, previous studies have found that gum disease raises the risk of premature birth. Aagaard speculates that if oral bacteria do reach the placenta through the blood, then it is possible that diseased and bleeding gums could allow harmful bacteria to reach and colonise the placenta, potentially triggering premature birth.

In a separate study in macaques, Aagaard’s team has shown that giving pregnant animals a high-fat diet altered their offspring’s microbiome. Many previous studies have shown that a person’s risk of obesity and heart disease is affected by their mother’s diet, but it was thought this was passed on through epigenetic mechanisms – chemical changes that switch the offspring’s geness on or off. “But layered on top of that are variations in the microbiome,” says Aagaard.

Journal reference: Science of Translational Medicine, DOI: 10.1126/scitranslmed.3008599

Clarification, 22 May 2014: Kjersti Aagaard is affiliated to Baylor College of Medicine as well as Texas Children’s Hospital. We have updated the article accordingly.