The risk figure of 1/270.
D000328:Adult; D000013:Congenital Abnormalities / Q000453:epidemiology*; D004314:Down Syndrome / Q000175:diagnosis / Q000196:embryology / Q000453:epidemiology*; D005260:Female; D006801:Humans; D008423:Maternal Age; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D018566:Pregnancy, High-Risk; D011296:Prenatal Diagnosis; D014481:United States / Q000453:epidemiology