Deviations from Normal Birth Weight and Autism Risk — California, 1989–2002

Background: Approximately 25,000 children born in California during 1989–2002 received services through the Department of Developmental Services (DDS) for autism, yet knowledge is limited regarding autism risk factors. Low birth weight (LBW) and preterm delivery have been linked to social and learning deficits among children; however, findings from studies of birth weight and autism are inconsistent.

Objectives: We evaluated associations between birth weight and autism risk in the California Department of Public Health statewide database, linking DDS and vital statistics data on singleton live births during 1989–2002 surviving to age 1 year.

Methods: Our study focused on children eligible for autism services after DDS evaluation (n = 23,913); all other children in the cohort (n = 7,125,777) served as control subjects. Sociodemographic factors, birth weight, and gestational age were obtained from birth certificate files. Logistic regression was used to calculate adjusted odds ratios (AORs) between autism and birth weight categories (reference, 2,500–3,999 g), controlling for maternal age, education, race, parity, sex, gestational age, year, and delivery payment type.

Results: Eleven percent of singletons were high birth weight (≥4,000 g), 3.9% LBW (1,500–2,499 g), 0.4% very LBW (VLBW, 1,000–1499 g), and 0.2% extremely LBW (ELBW, <1,000 g). AORs for autism were highest for ELBW (2.20; 95% confidence interval [CI], 1.80–2.70) and elevated for VLBW (1.20; 95% CI, 0.99–1.46), LBW (1.13; 95% CI, 1.06–1.21), and high birth weight (1.12; 95% CI, 1.08–1.16), compared with the referent.

Conclusions: Deviations from normal birth weight, particularly ELBW, were associated with an increase in the odds of autism among singleton children. Exploring this association is necessary to confirm birth weight as a predictor.