How many letters does the top answer for “an abnormally small head and underdeveloped brain” have?

The most common answer is microcephaly (12 letters).

Crossword clue: an abnormally small head and underdeveloped brain

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

microcephaly 12 letters wn-def

an abnormally small head and underdeveloped brain

having an abnormally small head and underdeveloped brain 83% similar → microcephalic, microcephalous