title: Letter: Twinning and neural-tube defects.
mesh_terms: D000757:Anencephaly / Q000453:epidemiology / Q000209:etiology / Q000235:genetics*; D001315:Australia; D002170:Canada; D004200:Diseases in Twins; D004677:Encephalocele / Q000453:epidemiology / Q000209:etiology / Q000235:genetics*; D005260:Female; D006720:Homozygote; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D008591:Meningomyelocele / Q000453:epidemiology / Q000209:etiology / Q000235:genetics*; D012744:Sex Ratio; D016135:Spinal Dysraphism / Q000235:genetics*; D014481:United States