title: Letter: Aetiology of anencephaly and spina bifida.
mesh_terms: D000757:Anencephaly / Q000209:etiology*; D005260:Female; D006801:Humans; D008423:Maternal Age; D011247:Pregnancy; D016135:Spinal Dysraphism / Q000209:etiology*; D014429:Twins, Dizygotic