BEIJING (AP) — Women in the Chinese capital in the final stage of pregnancy during the 2008 Beijing Olympics — when officials strictly controlled air pollution — gave birth to heavier babies than in years when the city was smoggier, a study said Wednesday.

The study, led by epidemiologist David Q. Rich of the University of Rochester Medical Center and published in the journal Environmental Health Perspectives, found that babies born to Beijing mothers in their eighth month of pregnancy during the 2008 Summer Games were on average 23 grams (0.8 ounce) heavier than those born either a year earlier or a year later.

It found no significant association for mothers in their first through seventh months.

Studies previously have linked pollution to birth weight, but did not pinpoint at what stage of pregnancy the association is greatest. The researchers in China and the U.S. used records of more than 83,000 full-term births to mothers in Beijing from 2007 to 2009.

Beijing halted construction, shut factories and cut the numbers of vehicles allowed on the roads for 47 days for the games, providing the basis for a natural experiment on the effects of pollution.

The authors suggested that pollution controls — even short-lived ones — can have positive health benefits.

"These findings not only illustrate one of the many significant health consequences of pollution, but also demonstrate that this phenomenon can be reversed," Rich said in a statement.

Professor Chen Yuyu of the Applied Economics Department at Peking University, who was not involved in the birth study, said it was interesting but not conclusive because there could have been other unobserved factors during the Olympics that contributed to the results.

Chen, who was the co-author of a 2013 report that linked heavy pollution from coal burning to shorter lives in northern China, said more research is needed to "improve our understanding of pollution's impact on people's health."