A GRADIENT RELATIONSHIP BETWEEN LOW-BIRTH-WEIGHT AND IQ AT AGE 6 YEARS

Objective: To test for a suspected adverse effect of low birth weight (less than or equal to 2500 g) on IQ at age 6 years in two socioeconom ically disparate populations. Design: Nonconcurrent prospective study. Setting: An urban, predominantly disadvantaged population and a subur ban middle-class population in southeastern Michigan. Participants: lo w-birth-weight (N=473) and normal-birth-weight (N=350) subjects, rando mly selected from the 1983 through the 1985 newborn lists of two major hospitals, one serving an urban and the other a suburban population. Main Outcome Measure: The Wechsler Intelligence Scale for Children-Rev ised to measure IQ. Results: The mean full-scale IQ of low-birth-weigh t children was 4.9 points lower than that of normal-birth-weight child ren, controlling for population site, maternal IQ, maternal education, and race (95% confidence interval [CI], 3.0 to 6.8). There was no evi dence of low-birth-weight interaction with population site, and the sa me IQ difference was detected in both populations. In the urban popula tion, low birth weight was associated with an increased risk for IQ be low 85 (odds ratio, 2.2; 95% CI, 1.3 to 3.7). In the suburban populati on, low birth weight was associated with an increased risk for IQ belo w 100 (odds ratio, 2.0; 95% CI, 1.2 to 3.4). A gradient relationship w ith full-scale IQ was observed, with the largest deficit in those born weighing 1500 g or less, an intermediate deficit in those born weighi ng 1501 through 2000 g, and the least pronounced deficit in chose born weighing 2001 through 2500 g. Conclusions: The overall effect of low birth weight represents an increase of approximately 10% of low-birthw eight children with an IQ of more than 1 SD below the population's mea n. Children born weighing 2000 g or less bore the major burden of the IQ deficits.