OBJECTIVE To investigate the association between severity of impairment and gestational age in unilateral and bilateral spastic cerebral palsy, and to determine whether the influence of gestational age is independent of deviations from optimal birth weight. STUDY DESIGN The study group was a United Kingdom cohort of 4772 cases of spastic cerebral palsy born between 1960 and 1997, with information on birth demographics and severity of impairment. Generalized additive models were used to determine the proportions of cases severely impaired, by gestational age, and to determine whether gestational age or deviations from optimal birth weight better predicts severity of impairment. RESULTS For unilateral spastic cerebral palsy, the proportions of severe impairments did not vary with gestational age. In contrast, for bilateral spastic cerebral palsy, the proportions of severe motor or cognitive impairments increased with increasing gestational age (e.g., from 20% to 50% between weeks 30 and 40 for cognitive impairment). For spastic cerebral palsy, gestational age is at least as good as deviation from optimal birth weight in predicting severity. CONCLUSIONS The severity of impairment increases with increasing gestational age in bilateral spastic cerebral palsy. This suggests differing etiologies in term and preterm infants and supports the theory that the developing brain is better able to compensate after a cerebral insult.