Of the placentas studied, Escherichia coli (E. coli) was the species with the highest abundance in most individuals, a bacterium that lives in the intestines of most healthy individuals.

Prevotella tannerae (gingival crevices) and non-pathogenic Neisseria species (mucosal special surfaces), both species of the oral cavity, were also detected in highest relative abundance.

"Interestingly, when we looked very thoroughly at the placenta in relation to many other sites of the body, we found that the placental microbiome ... is not much like the vaginal or intestinal microbiome, but rather is most similar to the oral microbiome," said Aagaard.

The researchers suspected that oral microbes might slip into the mother's bloodstream and end up in placenta.

The finding has important implications on the likely importance of oral health during pregnancy, she said. "It reinforces long- standing data relating periodontal disease to risk of preterm birth."

The mostly non-pathogenic bacteria living in the placenta have important day-to-day functions, like metabolizing vitamins at healthy levels for a developing fetus, they said.