A series of studies has suggested air pollution may be harmful during pregnancy, but the issue remains unresolved.
"We had an opportunity to use a unique data resource in New York City that was designed to estimate exposure throughout the city, which allowed us to improve on past studies, as well as examine a large, ethnically diverse population," David Savitz told Reuters Health in an email.
Savitz, from the Alpert Medical School of Brown University in Providence, Rhode Island, led the new study.
He and his colleagues looked at the birth weights of more than 250,000 babies born in New York City hospitals during 2008 to 2010.
Using data from the New York City Community Air Survey, they estimated how much pollution mothers had been exposed to during pregnancy based on women's home addresses.
The researchers focused on particulate matter less than 2.5 micrometers in diameter, known as PM2.5, and nitrogen dioxide.
They found birth weights dropped by about 48 grams (less than two ounces) for every 10 microgram per cubic meter increase in particulate matter women were exposed to throughout their pregnancies.
For each 10 parts per billion increase in nitrogen dioxide exposure throughout pregnancy, birth weight was reduced by 18 grams, Savitz's team reported in the American Journal of Epidemiology.
The U.S. Environmental Protection Agency (EPA) has set national standards of 15 micrograms of PM2.5 per cubic meter on average over the course of a year and 53 parts per billion of nitrogen dioxide.
Another report from the New York survey found average pollution levels were generally within those standards - although they were measured slightly differently - but varied widely.
The study doesn't prove air pollution was responsible for the slight decreases and there are many factors that can influence birth weight, the researchers noted.
The drops in birth weight don't point to any health concerns at this time, but the numbers do suggest a potential for problems, they said.
"While these are not important for any individual, on a population level, if a large number of births are shifted by that amount, there would be a real public health concern," Savitz said.
"The message really concerns policy, raising awareness that even though we have reduced air pollution levels substantially, there is still reason to believe that further reductions would be beneficial and quite possibly, have a small beneficial effect on the health of newborns," he said.
"It's really important to study air pollution because everybody's exposed to it and that means all pregnant women are exposed to it," Tracey Woodruff told Reuters Health.
She directs the Program on Reproductive Health and the Environment at the University of California, San Francisco, and wasn't involved in the new study.
The type of exposure measured in this study is not really something that can be avoided because these tiny particles easily travel indoors, Woodruff said. So the best way to reduce exposure is to address the sources.
This type of pollution generally comes from combustion sources, she said, like cars, trucks and power plants, and from agricultural sources.
"The EPA has been putting out new regulations related to particulate matter air pollution because PM is also associated with a whole host of adverse health outcomes including mortality and a number of respiratory and cardiovascular health problems," Woodruff said.