APA Dictionary of Psychology

an abnormal head due to a congenital defect. The head may be abnormally large (as in hydrocephalus), abnormally small (as in microcephaly), or square shaped (as in some cases of osteopetrosis), or it may have sutures that fail to close. The defect is often related to a chromosome abnormality. Chromosomes 17 and 18 are often involved through trisomy or other aberrations.