Heavy maternal workloads are considered to be hazardous to the fetus. The effects of physical activity during pregnancy on low birth weight (LBW), small for gestational age (SGA), and prematurity were assessed from a sample of 1797 women in a follow-up study at the antenatal clinic of two hospitals in southern Thailand. The women were interviewed twice, at 17 and 32 gestational weeks. Outcome data were obtained from medical records and the newborn gestational age determined using Dubowitz's score. The risk of SGA was elevated for women working >50 hours/week, squatting in work, commuting >1 hour/day, and having high psychological job demands; the risk of preterm delivery was increased with obstetrical complications. Women who worked long hours and had demanding work conditions had an elevated risk of giving birth to SGA infants but not of preterm delivery.