Studies in several countries have suggested an association between air pollution and low birth weight. In São Paulo, Brazil, a city with high levels of air pollution, an association was shown between air pollution and intrauterine mortality and late fetal loss, and umbilical cord blood concentrations of carbon monoxide correlated with ambient levels. Now another study in São Paulo has related air pollution to low birth weight.

Clinical and other data concerning all singleton, full term, live births in São Paulo during 1997 were collected from official certificates. Air pollution data were available from the São Paulo environmental agency. Maternal exposure to air pollutants was estimated for each trimester of pregnancy using city wide measurements. Of a total of 214 973 live births in the city in 1997 data were analysed for 179 460. The mean birth weight for singleton pregnancies was 3186 g and 5% were of low birth weight (<2500 g). Birth weight was affected by infant sex, maternal age, maternal education, number of antenatal visits, parity, and type of delivery. The effect on birth weight of exposure to air pollutants (respirable particles (PM₁₀), sulphur dioxide, carbon monoxide, nitrogen dioxide, and ozone) was calculated for each trimester with adjustment for the other factors affecting birth weight. Low birth weight was related to higher levels of exposure to PM₁₀ and carbon monoxide in the first trimester. A 10 μg/m³ increase in exposure to PM₁₀ was associated with a 13.7 g reduction in birth weight and a 1 ppm increase in carbon monoxide exposure was associated with a 23.1 g reduction.

Exposure to air pollutants in the first trimester of pregnancy may have an adverse effect on fetal weight gain.