A new paper in Environmental Health Perspectives suggests that high levels of air pollution may interfere with fetal development, leading to lower birth weights. The study, which uses data from the 2008 Olympics in Beijing, shows that babies who were born during or shortly after the Olympics were heavier than those born at the same time of year in 2007 or 2009.

During the 2008 Olympics, the Chinese government implemented measures to substantially limit Beijing’s air pollution, including restricting car and truck use, seeding clouds to create rain, and temporarily shutting down factories and construction sites. These measures dramatically improved Beijing’s famously poor air quality, reducing the levels of sulfur dioxide levels by 60 percent and levels of both carbon monoxide and nitrogen dioxide by over 40 percent. After the games the government relaxed these restrictions and pollution returned to normal.

The study focused on mothers who were in their eighth month of pregnancy during the Olympics and compared them to mothers who were pregnant at the same time in the years before and after the games. The mothers who were pregnant during the Olympics gave birth to babies who were on average 23 grams heavier than the babies born during non-Olympic years.

The researchers selected mothers in their eighth month of pregnancy because it is a crucial period for fetal development. Babies grow in size the most during the last stages of pregnancy, and the researchers speculate that the changes in birth weight they observed were caused by pollution interfering with this important period of growth.

The “natural experiment” caused by the temporary reduction of air pollution levels during the Olympics has also been used for another study by the same researchers. In that study they determined that pollution increases the risk of cardiovascular disease. The authors of both studies say that their research shows that even short-term pollution controls can improve public health.