A large US study suggests that exposure to fine air pollution, specifically PM2.5, during early pregnancy is linked to reduced baby birth weight. Researchers analyzed data from nearly 17,000 mother-baby pairs, finding that the earliest weeks of pregnancy, when major organs form, appear most sensitive to pollution. PM2.5, originating from vehicle exhaust and industrial emissions, can be inhaled deep into the lungs and its impact on birth weight was observed even at low exposure levels.