title: Letter: Twinning and illegitimacy in C.N.S. defects.
mesh_terms: D000328:Adult; D002490:Central Nervous System / Q000002:abnormalities*; D000013:Congenital Abnormalities / Q000453:epidemiology*; D005260:Female; D006801:Humans; D007086:Illegitimacy; D007231:Infant, Newborn; D008297:Male; D010298:Parity; D011247:Pregnancy; D011272:Pregnancy, Multiple; D012744:Sex Ratio; D014427:Twins; D014922:Wisconsin