Background: This study addresses children of parents with intellectual disability in Norway. The aim was to examine: (i) the impact of definitions of intellectual disability on prevalence, (ii) whether numbers were increasing, (iii) the prevalence of motherhood and fatherhood and (iv) rates of lost custody. Methods: Analyses of national registers (n = 30,834) and mapping in four municipalities (n = 85). Results: 0.19% of all children had parents with recorded intellectual disability, increasing to 0.87% with wider inclusion criteria. The number of children born to parents with intellectual disability has been declining since the mid-1980s. The proportion of mothers with intellectual disability was twice that of fathers. Parental custody was revoked for 30-50% of children, with single mothers being at particular risk. Parents with intellectual disability accounted for 20-25% of all custody cases. Conclusions: The results show that prevalence depends on the definition of intellectual disability. The decreasing number of children and the need for development of specially adapted family supports are discussed.