The discovery was made among lab animals but is likely to be valid for humans too, which opens up potential targets for drugs that could block the transmission pathway, they said.
The listeria germ causes flu-like symptoms and can lead to meningitis and convulsions if it invades the nervous system.
Unpasteurised dairy products, contaminated meats and unwashed raw vegetables are possible sources of the bacterium, which enters the blood system through the intestine.
For reasons not fully understood, pregnant women are 20 times likelier than other healthy adults to contract the disease, called listeriosis. It affects eight in a million people in the United States, and five per million in France.
Infection of a woman during pregnancy can cause miscarriage or stillbirth, and is often passed on to newborns, so scientists have sought to find out how the bacteria moves across the placenta.
A team led by Marc Lecuit of the Institut Pasteur in Paris identified two invasive proteins, called InlA and InlB, that are essential to the molecular mechanism that infects the foetus.
Lecuit found that, among lab animals, only gerbils shared the same duo of biochemical pathways as humans.
In experiments, pregnant gerbils were orally infected with listeria bacteria.
Fluorescent tracers in the bacteria showed that the disease reached and crossed the placenta.
The fatality rate amongst the pregnant gerbils was 100 percent, a result so extreme that the researchers decided to use other analytical tools, which did not involve killing animals, to corroborate the data.
"In a species in which both pathways are functional, both InlA and InlB are critical for placental invasion and foetal infection," the study concluded.
In a second set of experiments, mice were genetically modified to activate one of the two pathways that was naturally blocked, again producing the predicted results.
Lecuit cautioned that his findings would not result in "immediate applications."
"When we fully understand the mechanism of a disease, we can devise a way to block it," he said. "But for now, the best way to kill the bacteria is still antibiotics."
The study is published by the British-based science journal Nature.