Key PointsQuestionAfter adjustment for genetic factors, is birth weight associated with neurodevelopmental conditions?FindingsIn this case-control study of 393 twins in Sweden, the twin with a lower birth weight in monozygotic twin pairs, but not dizygotic pairs, had more autism and attention-deficit/hyperactivity disorder (ADHD) symptoms, lower IQ ratings, and higher odds of having a diagnosis of autism and ADHD compared with their co-twin.MeaningThese findings suggest that birth weight contributes to neurodevelopmental conditions when adjusting for genetic factors.