Pregnant women exposed to traffic pollution in their first trimester are more likely to give birth prematurely or have an underweight baby, according to new research.

Scientists at New York University, United States (US), exposed mice at various stages of pregnancy to air pollution. The vast majority of those exposed in their first trimester (83 percent) had premature babies.

Experts insist the study adds credence to the idea that mothers-to-be should avoid heavily polluted areas or fit pollution filters to their homes to protect their child.

The study was published in the journal Environmental Health Perspectives.

The study looked at the effects of fine particulate air pollution, which is made up of particles less than one ten-thousandth of an inch in diameter, or PM2.5. This invisible soot is breathed deeply into the lungs and has previously been linked to risk for asthma and heart disease.

Premature births and low birth weight also increases risk for vision and hearing problems, learning problems and even death. The study exposed mice at various stages of pregnancy to PM2.5 levels comparable to those found in highly-polluted urban environments.

It found exposure to air pollution during the equivalent of the first or second trimester in humans was linked to more negative birth outcomes than exposure later in pregnancy.

Previous studies had linked high levels particulate matter exposure to low birth weight, but impact of the timing of maternal exposure on birth weight had been debated.

The new findings suggested exposure during the first two trimesters has the greatest affect.