The relation between fetal growth and attention deficit hyperactivity disorder (ADHD) cuts across the normal range of birth weights suggesting that subtle variations in fetal development may influence brain and cognitive function. We investigated the relation of ADHD-related endophenotypes, such as the striatum morphology, motor response and inhibition, with birth weight and gestational age in healthy children. 157 Six-year-old boys born at term (37 to 41 weeks) within the normal range for birth weight (2500 to 4630 g) underwent magnetic resonance imaging (MRI) and performed the stop signal task. Linear regression was used to examine effects of birth weight, gestational age, and their interaction on striatal volumes and shapes as well as motor response and inhibition. Interactive effects of birth weight and gestational age, even within the normal range, predicted caudate volumes and shapes. Boys with relatively low birth weight and shorter gestation had smaller caudate volumes, reflected by shape contraction in the middle body, and in addition performed worst in motor response, reflected by mean reaction time and its variability. Our results supported the idea that prenatal influences on neurocognitive and brain development are not limited to the extreme range, but occur across the entire population. Variations in brain structure and cognitive endophenotypes associated with childhood ADHD psychopathology are sensitive to subtle prenatal influences, which provides guidance for intervention research to improve mental health of children.