Ischemia/hypoxia: The periventricular white matter of the neonatal brain is supplied by the distal segments of adjacent cerebral arteries. Although collateral blood flow from two arterial sources protects the area when one artery is blocked (e.g., thromboembolic stroke), this watershed zone is susceptible to damage from cerebral hypoperfusion (i.e., decreased cerebral blood flow in the brain overall). Since preterm and even term neonates have low cerebral blood flow, the periventricular white matter is susceptible to ischemic damage. Autoregulation of cerebral blood flow usually protects the fetal brain from hypoperfusion, however, it is limited in preterm infants due to immature vasoregulatory mechanisms and underdevelopment of arteriolar smooth muscles.