title: Letter: Sequential fetus-fetus interaction and C.N.S. defects.
mesh_terms: D000757:Anencephaly / Q000453:epidemiology*; D001722:Birth Order; D004200:Diseases in Twins; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D010298:Parity; D011247:Pregnancy; D012737:Sex Factors; D012744:Sex Ratio; D016135:Spinal Dysraphism / Q000453:epidemiology*; D014427:Twins