Neurological problems include apnea of prematurity, hypoxic-ischemic encephalopathy (HIE), retinopathy of prematurity (ROP),[23] developmental disability, transient hyperammonemia of the newborn, cerebral palsy and intraventricular hemorrhage, the latter affecting 25% of babies born preterm, usually before 32 weeks of pregnancy.[24] Mild brain bleeds usually leave no or few lasting complications, but severe bleeds often result in brain damage or even death.[24] Neurodevelopmental problems have been linked to lack of maternal thyroid hormones, at a time when their own thyroid is unable to meet postnatal needs.[25] Cardiovascular complications may arise from the failure of the ductus arteriosus to close after birth: patent ductus arteriosus (PDA). Respiratory problems are common, specifically the respiratory distress syndrome (RDS or IRDS) (previously called hyaline membrane disease). Another problem can be chronic lung disease (previously called bronchopulmonary dysplasia or BPD).