Hello Thank you for your query. It is rare to have an absent cavum septum pellucidum without any other brain anomalies. Absence of the CSP is associated with an extremely wide spectrum of neuroanatomic malformations: these range from the lethal entities of hydranencephaly and alobar holoprosencephaly; to the potentially serious but nonlethal entities of schizencephaly, porencephaly, basilar encephaloceles, severe hydrocephalus, and the less severe prosencephalic cleavage disorders (including syntelencephaly); to the normal variant, the rare and somewhat controversial entity of isolated septal deficiency. I am surprised why this wasnt discussed with you. You can have a repeat ultrasound as a second opinion, but if it is absent, it will not magically show up after 4 weeks. Discuss the prognosis in detail with your obstetrician and a neonatal neurologist. take care.