All women are offered screening for Downs syndrome. The purpose of the screening test is to decide which women should receive more invasive tests that would give a definitive diagnosis of Downs Syndrome.
It is their choice whether to go ahead with screening. These involve taking measurements, combining that with the mothers age (older mothers have a greater risk of Downs syndrome) and producing a risk score to calculate the risk of Downs syndrome.
“Combined Test” is the first line, most accurate and test of choice where possible
- Performed between after 11 and before 14 weeks gestation
- Ultrasound measures nuchal translucency (thickness of the back of the neck of the fetus – Downs Syndrome is a cause of thickness >6mm)
- Maternal blood tests:
- Beta‑Human Chorionic Gonadotrophin (beta-HCG) -a higher result indicates greater risk
- Pregnancy‑Associated Plasma Protein‑A (PAPPA) – a lower result indicates a greater risk
- Performed between 15 and 20 weeks gestation
- Only involves maternal blood test results:
- Beta-HCG -a higher result indicates greater risk
- Alpha-fetoprotein (AFP) – a lower result indicates a greater risk
- Serum oestriol (female sex hormone) – a lower result indicates a greater risk
- Performed between 15 and 20 weeks
- Identical to the triple test, but also includes maternal blood for:
- Inhibin-A – a higher result indicates a greater risk
Antenatal Testing for Downs Syndrome
The screening tests above provide a risk score for the fetus having Downs Syndrome. If the risk of Downs is greater than 1 in 150 (a result that 5% of those tested will get) then the woman is offered amniocentesis or chorionic villus sampling. These provide a sample of the fetal cells, that can then undergo karyotyping and give a definitive answer of whether they have Downs or not.
Chorionic villus sampling involves ultrasound guided biopsy of placental tissue. This is used when testing is done earlier in pregnancy (before 15 weeks).
Amniocentesis involves ultrasound guided aspiration of some amniotic fluid using a needle and syringe. This is later in pregnancy once enough amniotic fluid makes it safer to take a sample.