The month of onset, duration, and incidence of dental enamel hypoplasia and hypocalcification was determined in sub-adults from the Dickson Mounds (Illinois) skeletal series (950-1300). The onset of enamel defects occurred predominantly during the intrauterine period, suggesting maternal stress. There are marked differences in survivorship and the duration of enamel disruption in those affected prenatally and postnatally. The relationship between these data and studies of adult dentition is examined.