Women in Beijing in the final stage of pregnancy during the 2008 Olympics - when officials strictly controlled air pollution - gave birth to heavier babies than in years when the city was smoggier, a study has found.

The study, led by epidemiologist David Rich of the University of Rochester Medical Center, found that mothers in their eighth month of pregnancy during the Games had children which were born on average 23 grams 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.

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.

It led to memorable images of blue skies during the competition.

83,000 mothers studied

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 US used records of more than 83,000 full-term births to mothers in Beijing from 2007 to 2009.

"Late pregnancy is a particularly important period of foetal growth, as during this time the foetus experiences the greatest amount of physical growth, and the development of the central nervous, cardiovascular, and musculoskeletal systems accelerates," the study's authors said in a statement.

"The study suggests that pollution may be interfering with this period of development."

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

"We saw that there was this health benefit, presumed health benefit, in even these young folks," Rich told Al Jazeera on Wednesday.

"What was really nice about the Beijing Olympics is that we got to ... [go] against the way that we normally do these studies. Here we're going to see if decreases in pollution caused an increase in birth weight," he said.

"So, hopefully providing more convincing evidence ... that this is actually a real phenomenon."

The study was published in the journal Environmental Health Perspectives on Wednesday.