Non-clinical sciences/Genetics · Screening & counselling

Antenatal screening (Down's)

What it means

Combined test (11–14 wk): nuchal translucency + PAPP-A + free β-hCG + maternal age. Quadruple test (14–20 wk) if combined missed: AFP, unconjugated oestriol, β-hCG, inhibin A. Higher-chance result → offer NIPT (cell-free fetal DNA, very high sensitivity) or diagnostic CVS (11–14 wk)/amniocentesis (≥15 wk).

Worked example

In trisomy 21 the combined test typically shows ↑nuchal translucency, ↓PAPP-A, ↑β-hCG.

In the exam

At 12 weeks a screening test combines a nuchal-translucency scan with PAPP-A and β-hCG to estimate the chance of trisomy 21.

What settles it

Screening (combined/quad/NIPT) estimates CHANCE; only CVS/amniocentesis (invasive) gives a diagnostic karyotype.

Classically confused with

Genetic counselling principles

Source: NHS Fetal Anomaly Screening Programme