Screening for Down's syndrome

• 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. 
• All women are offered screening but it is their choice if they go ahead with it. The purpose is to decide which women should receive more invasive tests to establish a definitive diagnosis
•  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.
• Combined test:

  • First line and most accurate screening test
  • Performed between 11 and 13 weeks gestation and involves combining results from ultrasound and maternal blood tests
  • Ultrasound measures nuchal translucency - thickness of the back of the neck - greater than 6mm indicates Down's syndrome
  • Maternal blood tests:
  • Beta-human chorionic gonadotrophin (beta-HCG) - higher result indicates a greater risk
  • Pregnancy-associated plasma protein-A (PAPPA) - lower result indicates a greater risk
• 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.
• Invasive testing for karyotyping:

  • Chorionic villus sampling - ultrasound guided biopsy of the placental tissue - used before 15 weeks gestation
  • Amniocentesis - ultrasound guided aspiration of amniotic fluid using a needle and syringe - used later in pregnancy
• Risks of invasive diagnostic procedures:

  • Miscarriage
  • Infection
• Non-invasive prenatal testing:

  • Offered following a higher chance result following combined or quadruple tests
  • Assesses placental cell-free foetal DNA (found in bothers blood) and combines with the mother's background probability of trisomy
  • Still considered a screening test and any positive result needs to be confirmed with invasive testing
  • Suitable from 10 weeks gestation
• The combined test also assesses chance of foetus having Edward's syndrome (trisomy 18) and Patau's syndrome (trisomy 13)
• Quadruple test:

  • Only screens for Down's syndrome
  • offered during the second trimester when nuchal translucency cannot be obtained after 2 attempts or if measurements suggest gestation greater than 14+1 weeks
  • Alpha-fetoprotein - low
  • Beta-hCG - high
  • Inhibin-A - high
  • Unconjugated oestriol - low
  • Has a lower detection rate than combined test