Recent research suggests that fetal exposure to increased maternal body mass index (BMI) during pregnancy may be associated with psychopathology later in life. When this link first emerges, and if it is due to intrauterine exposures or confounding variables is not known. We therefore assessed associations between maternal pre-pregnancy BMI and: (1) temperament at 1 year of age, and (2) Child Behavior Checklist internalizing and externalizing scales at age 2 in the 2900 mothers and infants enrolled in the Western Australian Pregnancy Study. Pre-pregnancy BMI was positively associated with externalizing scores (β = 0.131, 95 % CI 0.013–0.249) at age 2, even after adjustment for confounders, but not with internalizing scores or an increased risk of difficult temperament. These data suggest that fetal exposure to increased maternal BMI is associated with elevated levels of behavior problems as early as age 2, and that this may be linked to the intrauterine environment.