What if a fetus has small head circumference?

Multiple causes: Probably the most common group is due to genetics & these kids grow up fine with normal development & intellectual functioning. The alternatives are many and have variable outcomes: pre-natal injury (toxins, trauma, infection, low oxygen); brain malformations;metabolic disorders ; syndromes :degenerative disorders. These groups have specific findings on evaluation but may not be detected until older.

Wait and see: You need to discuss with your pediatrician. Are length and weight small? Was the child premature? Are there any dysmorphic features or signs of a syndrome? You may have to wait a while as child develops to get an answer