Background: In 2011, the Swiss Society of Neonatology issued new guidelines for the care of preterm infants at the limit of viability leading to higher survival of infants born <28 weeks of gestation. It is unclear whether and how these recommendations affected the prevalence and severity of cerebral palsy (CP).

Objective: To investigate whether the prevalence, severity, and subtypes of CP in extremely preterm infants differ in two successive birth cohorts.

Methods: Retrospective, population-based analysis of prospectively collected data on infants born <28 weeks of gestation. CP prevalence, subtypes (Surveillance of cerebral palsy in Europe - SCPE classification), and severity (Gross Motor Function Classification System - GMFCS) assessed at 2 years corrected were compared between the two birth cohorts (2006-2011 and 2012-2017).

Results: Of 3244 registered infants, 2090 survived, of whom 1764 were followed up (84 %). Mortality was 38 % for the first period and 33 % for the second (p = 0.003) and 112 were diagnosed with CP. CP prevalence was 34.5 per 1000 live births (37.0 for 2006-2011 and 32.4 for birth-years 2012-2017, p = 0.476). A trend towards more bilateral spastic CP (2006-2011: 32 % and 2012-2017: 50 %, p = 0.055) and more severe cases (2006-2011: 14.3 % and 2012-2017: 24.9 %, p = 0.154) was observed in the second period. CP severity was associated with cystic periventricular leukomalacia (PVL) (OR 3.4, 95 %-CI 1.1-10.3, p = 0.033) and necrotizing enterocolitis (NEC) (OR 5.5, 95 % CI 1.2-25.1, p = 0.028) but not with other neonatal morbidities.

Conclusion: These results suggest that the greater number of bilateral forms and severe cases of CP could be due to the higher number of surviving infants in the 2011-2017 cohort. PVL and NEC are the factors mostly associated with severe cases of CP in Switzerland.