DHA and arachidonic acid (AA) are important for neurodevelopment. A traditional neonatal neurological examination and the evaluation of general movement quality are sensitive techniques for assessing neurodevelopment in young infants. Mildly abnormal general movements at 3 months have been associated with a non-optimal current brain condition. We investigated whether supplementation of DHA during pregnancy and lactation influences the infant's brain development and whether additional AA modulates this effect. Healthy women were randomly assigned to DHA (220 mg/d, n 42), DHA+AA (220 mg each/d, n 41) or control (n 36), from about week 17 (range 14–20 weeks) of pregnancy until 12 weeks postpartum. The control and the DHA+AA groups had approximately comparable dietary DHA/AA ratios. The standardised neonatal neurological examination was carried out at 2 weeks. General movement quality was assessed at 2 and 12 weeks. Neither DHA alone nor DHA+AA influenced outcomes in the traditional examination. General movement quality of infants in the DHA group was lower than that of infants in the other two groups, especially at 12 weeks: 61 % of the infants in the DHA group showed mildly abnormal general movements compared with 31 % in the control group (P = 0·008) and 34 % in the DHA+AA group (P = 0·015). We conclude that general movement quality at 12 weeks is sensitive to the maternal dietary DHA/AA balance.