Microcephaly: An abnormally small head due to failure of brain growth. Microcephaly is an ominous sign because it is almost always associated with developmental delay and mental retardation. Many factors can impair the growth of the brain, including intrauterine infections (such as rubella, cytomegalovirus, and toxoplasmosis), intrauterine chemical exposure (such as in fetal alcohol syndrome), excessive radiation exposure (as in an atomic bomb explosion), chromosome abnormalities (such as trisomy 13 and trisomy 18), and genetic syndromes (such as Fanconi syndrome and Williams syndrome). Recently, infection with the Zika virus during pregnancy has been associated with microcephaly in the newborn. The opposite of microcephaly is macrocephaly.
United States Centers for Disease Control and Prevention. "Zika Virus." Jun 27, 2016.