Risk of mental retardation among children born with birth defects.

BACKGROUND A paucity of epidemiologic research exists concerning the co-occurrence of birth defects and mental retardation (MR). Study of this co-occurrence may yield important clues about the causes of both. OBJECTIVE To examine the co-occurrence of birth defects and MR, taking into consideration the type of birth defect, level of MR, co-occurrence of MR with other developmental disabilities, and individual and maternal factors. DESIGN A retrospective cohort study of infants born in the California Central Valley with and without a structural birth defect by 1 year of age, and with or without MR by 7 to 9 years of age. SETTING AND PARTICIPANTS One-year survivors (N = 119 556) born in nonmilitary hospitals in 8 California counties between January 1, 1992, and December 31, 1993, for whom information about birth defects was recorded within the first year of life. MAIN OUTCOME MEASURE Diagnosis of MR by age 7 years considered as being mild or severe and as occurring without other developmental disabilities (isolated MR) or as occurring with other developmental disabilities, including cerebral palsy, epilepsy, or a pervasive developmental disorder. RESULTS Children with birth defects were nearly 27 times more likely to have MR by 7 years of age compared with children without a diagnosed birth defect regardless of type of defect (prevalence ratio, 26.8; 95% confidence interval, 22.7-31.7). Among those with birth defects, children with Down syndrome (prevalence ratio, 211.7; 95% confidence interval, 171.3-261.5) and children with sex chromosomal defects (prevalence ratio, 57.4; 95% confidence interval, 23.7-138.6) were at the highest risk for MR. Children with nonchromosomal defects, including central nervous system defects and all types of organ and system defects, were at substantially increased risk for all levels of MR. Risks of MR among children with Down syndrome and nonchromosomal defects were not substantially altered when adjusted for individual and maternal factors. CONCLUSIONS Children with chromosomal and other structural birth defects are at a substantially increased risk for having MR by 7 years of age compared with children born without a birth defect. Children with birth defects are at an especially increased risk for having severe MR and MR occurring independently of other developmental disabilities.