The causal effect of being born extremely preterm or extremely low birthweight on neurodevelopment and social‐emotional development at 2 years

To assess the causal effect of being born extremely preterm (EP; <28 weeks' gestation) or extremely low birthweight (ELBW; <1000 g), compared with being born at term, on neurodevelopment and social-emotional development at 2 years' corrected age.Prospective geographical cohort study of children born EP/ELBW over 12 months in 2016 from Victoria, Australia, and term-born controls. Children were assessed at 2 years' corrected age with the Bayley Scales of Infant and Toddler Development-3rd edition and the Infant-Toddler Social and Emotional Assessment. Delay was defined as <-1 standard deviation relative to the mean of controls. The estimand of interest was the mean difference/odds ratio (OR) between the EP/ELBW and control groups estimated using linear/logistic regression, adjusted for multiple pregnancy and social risk.A total of 205 EP/ELBW and 201 controls were assessed at 2 years. Delay/concerns were more common in the EP/ELBW group compared with controls, for cognitive (OR 3.7 [95% confidence interval 2.3, 6.0]), language (5.3 [3.1, 9.0]) and motor (3.9 [2.3, 6.3]) development, and social-emotional competence (4.1 [1.6, 10.2]).Being born EP/ELBW has an adverse effect on cognitive, language and motor development, and social-emotional competence at 2 years' corrected age. Close developmental surveillance, including social-emotional development, is recommended.