Down’s syndrome is a condition caused by three copies of chromosome 21. It is also called trisomy 21. It gives characteristic dysmorphic features and has many associated conditions. The extent to which a person is affected and the related conditions they have vary between individuals.
All women are offered screening for Down’s syndrome during pregnancy. The purpose of the screening test is to decide which women should receive more invasive tests to establish a definitive diagnosis.
It is the choice of the woman whether to go ahead with screening. The screening tests involve taking measurements from the fetus using ultrasound, combining those measurements with the mother’s age and blood results and providing an indication of the risk of Down’s syndrome. Older mothers have a higher risk of Down’s syndrome.
The combined test is the first line and the most accurate screening test. It is performed between 11 and 14 weeks gestation and involves combining results from ultrasound and maternal blood tests.
Ultrasound measures nuchal translucency, which is the thickness of the back of the neck of the fetus. Down’s syndrome is one cause of a nuchal thickness greater than 6mm.
Maternal blood tests:
- Beta‑human chorionic gonadotrophin (beta-HCG) – a higher result indicates a greater risk
- Pregnancy‑associated plasma protein‑A (PAPPA) – a lower result indicates a greater risk
The triple test is performed between 14 and 20 weeks gestation. It only involves maternal blood tests:
- 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
The quadruple test is performed between 14 and 20 weeks gestation. It is identical to the triple test, but also includes maternal blood testing for inhibin-A. A higher inhibin-A indicates a greater risk.
Antenatal Testing for Down’s Syndrome
The screening tests provide a risk score for the fetus having Down’s syndrome. When the risk of Down’s is greater than 1 in 150 (occurs in around 5% of tested women), the woman is offered amniocentesis or chorionic villus sampling.
These tests involve taking a sample of the fetal cells to perform karyotyping for a definitive answer about Down’s:
- Chorionic villus sampling (CVS) involves an ultrasound-guided biopsy of the placental tissue. This is used when testing is done earlier in pregnancy (before 15 weeks).
- Amniocentesis involves ultrasound-guided aspiration of amniotic fluid using a needle and syringe. This is used later in pregnancy once there is enough amniotic fluid to make it safer to take a sample.
Non-Invasive Prenatal Testing
Non-invasive prenatal testing (NIPT) is a relatively new test for detecting abnormalities in the fetus during pregnancy. It involves a simple blood test from the mother. The blood will contain fragments of DNA, some of which will come from the placental tissue and represent the fetal DNA. These fragments can be analysed to detect conditions such as Down’s.
NIPT is not a definitive test, but it does give a very good indication of whether the fetus is affected. NIPT is gradually being rolled out in the NHS as an alternative to invasive testing (CVS and amniocentesis) for women that have a higher than 1 in 150 risk of Down’s syndrome.
Last updated September 2020