Hunter Medical Research Institute scientist Adam Collison who ran the study called it an "amazing finding".
"Ultimately, it means that kids will grow up without asthma who would have had it," Dr Collison said.
The breath test measured levels of nitric oxide, a marker of lung inflammation.
In the study, half the participants were prescribed medication doses based on how the women assessed their own symptoms.
The other half kept track of how they felt, but they were also assessed more precisely using the breath test.