In a large, prospective study, mothers who smoked during one pregnancy but not another had smaller infants in the pregnancy in which they smoked, irrespective of birth order and many other factors that affect fetal growth. Light cigarette smokers gained an average 90 g less at term than non-smokers, entirely due to the smaller size of the light smokers' newborn infants. Heavy smokers gained 533 g less than non-smokers, only one-third of which was due to the smaller size of the heavy smokers' newborn infants. As smoking increased, placentas enlarged and developed microscopic lesions characteristic of underperfusion from the uterus. This underperfusion was probably periodic rather than continuous because the smokers' decidua had few of the arterial lesions that are characteristic of chronic low blood flow. Pregnancies were a mean 1.5 days shorter in smokers than in non-smokers, due to more frequent amniotic fluid infections in the smokers.