Curriculum · Screening (National/International)
Antenatal screening for Down syndrome
What it is
Down syndrome, written on the screening list as trisomy 21, is looked for during pregnancy by combined screening in the first trimester or by NIPT. The people screened are pregnant women, and the timing given is the first trimester. It sits alongside the other tests offered in pregnancy on the same list: HBsAg in every pregnancy, and the oral glucose tolerance test at 24 to 28 weeks gestation.
Causes and risk
Three modes of inheritance are given. Non-disjunction is the commonest. Robertsonian translocation, usually onto chromosome 14, is the second, and where one parent carries a balanced translocation in which both copies of chromosome 21 are fused together the recurrence is 100%. Mosaicism is the third.