BACKGROUND: The effects of in utero exposure to atypical antipsychotics on infant birth weight are unknown. AIMS: To determine whether atypical and typical antipsychotics differ in their effects on birth weight after maternal exposure during pregnancy. METHOD: Prospective data on gestational age and birth weight collected by the National Teratology Information Service for infants exposed to typical (n=45) and atypical (n=25) antipsychotics was compared with data for a reference group of infants (n=38). RESULTS: Infants exposed to atypical antipsychotics had a significantly higher incidence of large for gestational age (LGA) than both comparison groups and a mean birth weight significantly heavier than those exposed to typical antipsychotics. In contrast those exposed to typical antipsychotics had a significantly lower mean birth weight and a higher incidence of small for gestational age infants than the reference group. CONCLUSIONS: In utero exposure to atypical antipsychotic drugs may increase infant birth weight and risk of LGA.