Co-occurrence of birth defects and intellectual disability.

This study used population-based databases to ascertain birth defects and intellectual disability (ID), defined as full IQ < 70, in children born in Western Australia during 1980-99. Of the children surviving to 1 year (n = 474 285), 4.9% had birth defects and 1.3% ID. ID was identified in 7.9% of children with birth defects. After adjusting for sex, mother's age, race, parity, plurality, birthweight and gestational age the prevalence ratio (PR) [95% confidence interval (CI)] for ID in children with birth defects compared with those with no birth defects was 7.6 [7.2, 8.0]. Those with chromosomal anomalies comprised 3.2% of the group with birth defects. The percentage ID (and PR [95% CI]) in specific categories were: Down's syndrome 97% (84.5 [79.4, 90.0]), sex chromosome anomalies 30.3% (31.0 [23.8, 40.3]), other chromosomal anomalies 64.2% (54.2 [47.2, 62.3]). Birth defects were categorised according to system in the 96.8% of children with non-chromosomal anomalies. The percentage with ID (and PR [95% CI]) for birth defects in each system were: spina bifida 18.8 (16.7 [12.2, 23.0]); nervous (except spina bifida) 38.6 (33.4 [30.3, 36.9]); cardiovascular 4.2 (4.1 [3.5, 4.8]); gastro-intestinal 2.2 (2.0 [1.5, 2.7]); urogenital 2.6 (2.4 [2.0, 2.8]; musculo-skeletal 3.6 (4.0 [3.5, 4.6]); other non-chromosomal 7.0 (7.3 [6.5, 8.3]); and multiple systems 12.3 (10.2 [8.6, 12.2]). Birth defects were present in 30.2% of children with ID (27.7% of children with mild/moderate ID (IQ 40-69) and 54% of children with severe ID (IQ < 40)). Adjusted PRs for birth defects in children with any ID, mild/moderate ID and severe ID compared with children with normal intellectual function were 6.0 [5.8, 6.3], 5.5 [5.3, 5.8] and 10.5 [9.7, 11.4] respectively. The data are useful for those providing services for children with developmental disabilities especially for predicting family support and respite and accommodation requirements for children and adults with severe ID.