The placenta, which was long believed to be a sterile environment, is home to a community of microbes similar to those found in the mouth, researchers have discovered.
These microbes do not directly cause diseases. However the authors believe that changes to their community, referred to as a 'microbiome', might be a cause of common pregnancy complications such as preterm birth.
In the study, published in Science Translational Medicine, the researchers collected placental tissue from 320 women after delivery and analysed the DNA using an approach called shotgun metagenomics. This analysis revealed a gene expression pattern that suggested that a unique population of microbes lived within the placenta.
'We think this may be early evidence that the infant microbiome is populated before birth', said Dr Kjersti Aagaard, an associate professor of obstetrics and gynaecology at Baylor College of Medicine in Houston, USA, and a study author, told Bloomberg. 'We don't have direct evidence but it suggests that'.
The scientists also compared the composition of placental microbes to microbiomes found elsewhere in the mother's body. Such comparisons revealed strong similarities to the microbiome of the mouth. The authors suggest that microbes from the mouth are able to enter the bloodstream and travel to the placenta.
Certain types of microbe were more common in women who had given birth prematurely, a finding which supports previous studies where gum disease appears to raise the risk of preterm birth.
Dr 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.
Dr Aagaard is now beginning a larger study to explore this link by analysing the oral and placental microbiomes of more than 500 pregnant women at risk of preterm birth.
Professor Martin Blaser, director of the human microbiome programme at New York University, who was not an author on this paper, told the New York Times: 'I'm intrigued by the findings about the mouth and also the relationship with preterm labor'.
He added that pregnant women were often given antibiotics, 'for all kinds of reasons, many justified, but there's a slippery slope'.
'Assuming that the placenta was sterile anyway, doctors thought antibiotics would not affect the fetus. But if the placenta is not sterile, and is instead a portal for bacteria from the mother, what are the antibiotics doing?'