Cerebral Palsy Among Term and Post-term Births

Preterm birth is a strong predictor of cerebral palsy (CP), with risk of CP increasing with earlier delivery. There is a lower individual risk of CP among infants born at term, but even so, most infants with CP are born after 36 weeks of gestation. Little data are available within this range of term births on the association between risk of CP and gestational age in infants born at term. This population-based study investigated whether risk of CP is associated with timing of birth in the term and post-term period. Information obtained from the national health and insurance registries in Norway was used to identify 1,682,441 singleton live births between 1967 and 2001 with a gestational age of 37 to 44 weeks. Linkage to other national registries provided follow-up data for this cohort through 2005. The primary study outcome measure was the absolute and relative risk (RR) of CP among children who survived to at least 4 years of age. The gestational age was estimated using information based on the last menstrual period. Data from the Norwegian compulsory health insurance system identified 1938 term and post-term children with CP (1.15/1000 births). The lowest risk of CP was found among infants delivered at 40 weeks; the prevalence was 0.99/1000, with a 95% confidence interval (CI) of 0.90–1.08. Compared with term delivery, CP risk was higher with both earlier or later delivery: prevalence at 37 weeks was 1.91/1000 (95% CI, 1.58–2.25) with a RR of 1.9 (95% CI, 1.6–2.4); at 38 weeks, 1.25/1000 (95% CI, 1.07–1.42) with a RR of 1.3 (95% CI, 1.1–1.6); at 42 weeks, 1.36/1000 (95% CI, 1.19–1.53) with a RR of 1.4 (95% CI, 1.2–1.6); and after 42 weeks, 1.44 (95% CI, 1.15–1.72) with a RR of 1.4 (95% CI, 1.1–1.8). The association of CP risk with gestational age was even stronger with ultrasound-based gestational age than with last menstrual period for infant subgroups born at 37 weeks (prevalence, 1.17/1000 [95% CI, 0.30–2.04] with a RR of 3.7 [95% CI, 1.5–9.1]) and at 42 weeks (prevalence, 0.85/1000 [95% CI, 0.33–1.38] with a RR of 2.4 [95% CI, 1.1–5.3]). The RRs for gestational age were unaffected by adjustment for infant gender, maternal age, and several socioeconomic measures. These findings show that risk of CP is lowest at 40 weeks of gestation and is higher before and after this.