Background. Several studies have found an association between indicators of fetal growth and/or obstetric complications and schizophrenia but only a few studies have investigated the possible association between these factors and bipolar disorder. Furthermore, the results of these studies have been contradictory. The aim of this study was to investigate whether the risk of bipolar disorder is associated with exposure to indicators of fetal growth.

Method. A national population nested case-control study based on Danish longitudinal register databases was carried out. Conditional logistic regression was used, controlling for potential confounding factors such as parental age at birth, socio-economic indicators and psychiatric history. We identified 196 cases, and each case was time-, age- and sex-matched with 25 normal population-based controls. All cases were between the ages of 12 and 26 years at the time of diagnosis, were born between 1973 and 1983 and were admitted and diagnosed between 1987 and 1999.

Results. During the study period 1973–1983, none of the individual variables available for analyses (birthweight, birth length, gestational age and number of previous pregnancies in the mother) was associated with receiving a diagnosis of bipolar disorder.

Conclusions. None of the indicators of fetal growth under study could be identified as risk factors for bipolar disorder, suggesting that the etiologies of schizophrenia and bipolar disorder, at least in part, are different.