Context: Studies linking birth weight and mental illness onset are inconclusive. They have primarily focused on the World Health Organization low birth weight threshold (2500 g) and schizophrenia. To our knowledge, low birth weight per se has not been conclusively linked with schizophrenia risk and specificity of the effect to birth weight below the standard threshold or to particular psychiatric diagnoses has not been demonstrated.
Objectives: To examine whether (1) low birth weight (<2500 g) is associated with increased risk for adult schizophrenia; (2) risk extends into the normal weight range; and (3) risk is confined to schizophrenia or linked to other adult mental illnesses.
Design: Population-based cohort study.
Setting: Sweden and Denmark.
Participants: Singleton live births in Sweden (1973-1984) and Denmark (1979-1986) (N = 1.49 million). Births were linked to comprehensive national registers of psychiatric treatment, with follow-up to December 31, 2002 (Sweden), or to June 30, 2005 (Denmark). There were 5445 cases of schizophrenia and 57 455 cases of any adult psychiatric disorder.
Main outcome measure: Crude and adjusted odds ratios for birth weight less than or more than 3500 to 3999 g in consecutive 500-g strata (from 500-1499 g to > or =4500 g) for schizophrenia, any psychiatric diagnoses, and specified psychiatric diagnoses.
Results: Schizophrenia was associated with birth weight less than 2500 g. The association was not restricted to birth weight less than 2500 g and there was a significant linear trend of increasing odds ratios with decreasing birth weight across the birth weight range. This was mirrored for any psychiatric diagnosis and for each of the categories of psychiatric disorder.
Conclusions: Findings suggest there is an association between birth weight and adult mental disorder, but there is no indication this effect is specific to birth weight less than 2500 g or to schizophrenia. Future research should explore common disorder-specific mechanisms that may link birth weight to development of psychiatric disorder in adulthood.