Ambient air pollution and adverse birth outcomes: a natural experiment study.

Background: Radical regulations to improve air quality, including traffic control, were implemented prior to and during the 2008 Beijing Olympic Games. Consequently, ambient concentrations of nitrogen dioxide (NO2) and particular matter 10 micrometers or less (PM10), were reduced in a distinct and short window of time, which presented a natural experiment for testing the relationships between maternal exposure to PM10 and NO2 during pregnancy and adverse birth outcomes.

Methods: We estimated the effect of PM10 and NO2 exposure during each trimester of gestation on the risk of preterm birth among live births and the birth weight among term babies. The data were based on 50,874 live births delivered between January 1, 2006 and December 31, 2010 at the Beijing Haidian Maternal and Child Health Hospital. Air monitoring data for the same period were obtained from the Beijing Municipal Environmental Monitoring Center.

Results: Among full-term births, maternal exposure to NO2 in the third trimester predicted birth weight, with each 10-unit increment (per 10 ug/m(3)) in NO2 concentration associated with a 13.78 g (95 % confidence interval: -21.12, -6.43; p < 0.0001) reduction in birth weight. This association was maintained after adjusting for other pollutants, including carbon monoxide (CO), sulfur dioxide (SO2), and PM10. No relationship was found between the concentration of PM10 and low birth weight among full-term births. Neither PM10 nor NO2 concentrations predicted the risk of premature birth.

Conclusions: Exposure to ambient air pollution during certain periods of pregnancy may decrease birth weight, but the effect size is small.