Because of the high incidence of early childhood caries (ECC), a longitudinal study to identify risk factors from the prenatal period to the child's first birthday among 9- to 18-month-old children was conducted with negative binomial modeling. Overall, 495 children had dental examinations at ages 9, 12, and 18 months. Mothers were interviewed during the 2nd trimester of pregnancy and when the children had dental examinations. The highest incidence of caries was found among children who were born to mothers with >or= 10 decayed teeth and who never received calcium supplements during pregnancy, and children who were not fed supplementary foods at age 3 months, had sweet-tasting foods at 5 months, started snacking at 5 months, had sugary snacks, had soft drinks, and did not have their teeth brushed daily at 9 months. Thus, prenatal care and child-rearing-practices during and after birth are important risk factors for the incidence and incremental rate of ECC.