Surprising new research shows a small but diverse community of bacteria lives in the placentas of healthy pregnant women, overturning the belief that fetuses grow in a sterile environment.
These are mostly varieties of "good germs" that live in everybody. But Wednesday's study also hints that the make-up of this microbial colony plays a role in premature birth.
"It allows us to think about the biology of pregnancy in different ways than we have before, that pregnancy and early life aren't supposed to be these totally sterile events," said lead researcher Dr Kjersti Aagaard of Baylor College of Medicine in Houston.
All humans share their bodies with trillions of microbes on the skin, in the gut and in the mouth. These communities are called the microbiome. Many bacteria play critical roles in keeping us healthy. The Human Microbiome Project in the US mapped what made up these colonies and calculated that healthy adults cohabitated with more than 10,000 species.
Healthy newborns pick up some from their mother during birth but there have been some signs that the process could begin in-utero.
"We have traditionally believed in medicine that the uterus is a sterile part of the human body," said Dr Lita Proctor of the National Institutes of Health, who oversaw the microbiome project.
With the new research, "we realise that microbes may play a role even in fetus development", added Proctor, who was not involved in the work.
"The results of this study now open up a whole new line of research on maternal and paediatric health."
Aagaard's team earlier had studied the microbiome of the vagina, and learned that its composition changes when a woman becomes pregnant.
However, the most common vaginal microbes were not the same as the earliest gut bacteria that scientists were finding in newborns. What else, Aagaard wondered, could be "seeding" the infants' intestinal tract?
With colleagues from Baylor and Texas Children's Hospital, Aagaard analysed 320 donated placentas, using technology that teases out bacterial DNA.
The placenta isn't teeming with microbes. It harbours a low level, including the kinds of E. coli that live in the intestines of most healthy people.
But to Aagaard's surprise, the placental microbiome most resembled bacteria frequently found in the mouth, she reported in the journal Science Translational Medicine. The theory is that oral microbes slip into the mother's bloodstream and make their way to the placenta.
Aagaard said there appeared to be a role for different microbes. Some metabolised nutrients. Some were toxic to yeast and parasites. Some acted like natural versions of medications used to stop preterm contractions, she said.
In fact, among the 89 placentas that were collected after premature births, levels of some of the apparently helpful bacteria were markedly lower, she said.
Aagaard is now beginning a larger study to explore the link.