The aim of this study was to identify risk factors for failure to thrive (FTT) or weight faltering according to age of onset. The study is part of a Danish longitudinal population study of early risk mechanisms in child psychiatric disorders, The Copenhagen Child Cohort, which consists of a birth cohort of 6090 children born during the year 2000 and followed prospectively from birth. Weight faltering/FTT was defined as slow conditional weight gain, and divided into subtypes according to age of onset in the first year of life: birth to 2 weeks, 2 weeks to 4 months, and 4-8 months. Regardless of the age of onset, slow weight gain was found to be strongly associated with feeding problems, but the risk factors involved differed according to age of onset. Thus, onset within the first weeks of life clearly differed from faltering later on, the former being strongly associated with low birthweight and gestational age, with single parenthood and with mother having smoked during pregnancy. Onset between 2 weeks and 4 months was associated with congenital disorders and serious somatic illness, and with deviant mother-child relationship, whereas, onset between 4 and 8 months seemed to represent a group of children with feeding problems arising de novo in otherwise healthy children. In conclusion, weight faltering in infancy is clearly associated with contemporary measured feeding problems, but the risk mechanisms involved differ in early vs. late onset.