The NICHD Neonatal Research Network's Follow-Up study is a multi-center cohort study in which surviving extremely low birth-weight infants undergo neurodevelopmental, neurosensory and functional assessments at 22-26 months corrected age (Infants born prior to July 1, 2012 were seen at 18-22 month corrected age). The goal of the study is to identify potential maternal and neonatal risk factors that may affect infant neurodevelopment, including: