Background: Prematurity has been shown to be associated with an increased risk of intellectual disability (ID). Method: The aim was to establish whether the prevalence of ID, defined as significant limitations in both intellectual (intelligence quotient below 70) and adaptive functioning among moderately preterm (MP; 32[superscript + 0]-33 [superscript + 6] weeks) and late preterm (LP; 34[superscript + 0]-36[superscript +6 weeks) infants, is increased compared with that in term infants (=37[superscript +0] weeks). Antenatal and neonatal risk factors for ID among gestational age groups were sought. The national register study included all live-born infants in Finland in 1991-2008, excluding those who died before one year age, or had any major congenital anomaly or missing data. A total of 1 018 256 infants (98.0%) were analysed: very preterm (VP; <32[superscript +0] weeks, n = 6329), MP (n = 6796), LP (n = 39 928) and term (n = 965 203). Results: By the age of seven years, the prevalence of ID was 2.48% in the VP group, 0.81% in the MP group, 0.55% in the LP group and 0.35% in the term group. Intracranial haemorrhage increased the ID risk in all groups. Male sex and born small for gestational age predicted an increased risk in all but the MP group. Conclusions: The prevalence of ID decreased with increasing gestational age. Prevention of intracranial haemorrhages may have a beneficial effect on the neurodevelopmental outcomes of neonates.