The human placenta is not as sterile as once thought, but rather harbors a small but diverse group of microbes that may influence the course of pregnancy, U.S. researchers said Wednesday.

The findings, published in the U.S. journal Science Translational Medicine, hinted at an association between the composition of the placental microbial community known as microbiome and preterm birth, or birth of a baby at less than 37 weeks gestational age.

"After we completed our studies of the vaginal microbiome in pregnancy, we noted that the most abundant microbes in the mom's vagina were not what populated the baby's intestinal microbiome," lead author Kjersti Aagaard, associate professor of Baylor College of Medicine in Houston, Texas, said in a statement. "We reasoned that there must be another source 'seeding' the infant's gut at birth, so we sought to examine the placenta."

Aagaard and her colleagues characterized the bacteria in more than 300 placentas and found out the long-held notion that the placenta is a sterile environment is wrong.

"The placental microbiome is low in terms of microbial abundance but not as sterile as we previously thought," said Aagaard.