What does it mean ihf-0,8cm on head ultrasound in 5 months old baby and what are the consequences ?

Fluid: Interhemispheric fissure (IHF) is measured when US is done for large head size most of the time. It allows one to determine whether enlarging head size is due to fluid outside the ventricles vs inside. If the US did not show fluid inside enlarged ventricles and did show an upper limit of normal IHF, the cause of enlarging head is likely benign. 6-8 mm is the normal range http://radiopaedia.org/articles/benign-enlargement-of-the-subarachnoid-space-in-infancy