Context-While preterm delivery is a well-established risk factor for cerebral palsy (CP), preterm deliveries contribute only a minority of affected infants. There is little information on the relation of CP risk to gestational age in the term range, where most CP occurs.Objective-To determine whether timing of birth in the term and post-term period is associated with risk of CP.Design, Setting, and Participants-Population-based follow-up study using the Medical Birth Registry of Norway to identify 1,682,441 singleton children born 1967-2001 with a gestational age of 37-44 weeks and no congenital anomalies. The cohort was followed through 2005 by linkage to other national registries.Main Outcome Measures-Absolute and relative risk of CP for children surviving at least to 4 years of age.Results-Of the cohort of term and post-term children, 1,938 were registered with CP in the National Insurance Scheme. Infants born at 40 weeks had the lowest risk of CP, with a prevalence of 0.99 per thousand (95% confidence interval [CI], 0.90-1.08). Risk for CP was higher with earlier or later delivery, with relative risk reaching 1.9 at 37 weeks (95% CI, 1.6-2.3) and 1.4 at 42 weeks (95% CI, 1.2-1.6). These associations were even stronger in a subset with gestational age