title: Letter: Multiple ovulation and risk of anencephaly and spina bifida.
mesh_terms: D000757:Anencephaly / Q000453:epidemiology / Q000209:etiology*; D005260:Female; D006801:Humans; D007086:Illegitimacy; D007231:Infant, Newborn; D008423:Maternal Age; D010060:Ovulation; D011247:Pregnancy; D011272:Pregnancy, Multiple; D012959:Socioeconomic Factors; D016135:Spinal Dysraphism / Q000453:epidemiology / Q000209:etiology*; D014427:Twins