Pregnant women may get ill from Listeria at lower doses than previously thought, U.S. researchers found.

The risk of fetal or infant mortality among pregnant women who consume food containing 1 million cells of the food-borne pathogen Listeria monocytogenes in soft cheeses and other food is estimated at about 50 percent, suggesting five stillbirths potentially could occur when 10 pregnant women are exposed to that amount.

A previous risk assessment estimated more than 10 trillion cells would result in stillbirths to 50 percent of pregnant women exposed, researchers said.

"We’re not saying there’s a new epidemic here, we’re suggesting we’ve come up with a more accurate method of measuring the risk and how this deadly bacteria impacts humans, especially the most medically vulnerable among us," study co-author Mary Alice Smith of the University of Georgia said in a statement.

When estimates are extrapolated from data in tests on laboratory animals, the results showed "Listeriosis is likely occurring from exposure to lower doses than previously estimated," Smith said.