Screening for Down's syndrome

Down's syndrome is a condition caused by the presence of an extra chromosome in a baby's cells. It occurs by chance at conception and is irreversible, it is not caused by anything either parent has done. There is no such thing as a typical person with Down's syndrome, they vary a lot in appearance and ability, but are at an increased risk of health problems such as learning difficulties and heart and bowel problems. It is not possible to tell how much a baby will be affected when they grow up. All pregnant women should be offered screening for Down's syndrome. These tests can show if there is an 'increased chance' or 'higher risk' of your baby having Down's syndrome. Agreeing to a screening test does not mean you have decided what to do if a problem is found. Some couples simply prefer to know in order to be prepared, whilst others choose to end their pregnancy. More @How is my baby going to be screened for Down's syndrome and fetal anomalies?

The tests: the National Screening Committee (NSC) of UK recommends screening is available early in pregnancy. The test offered does depend on how early we see you in your pregnancy. A combination of blood tests and an ultrasound scan are available. Your doctor will give you more information about the tests available locally.

• Blood Test: a sample is taken from the mother and sent to the laboratory to measure the levels of chemicals or substances naturally found in your blood and those passed from the baby.
• Nuchal Translucency (NT) scan: this is a special ultrasound scan done between 11 and 13 weeks. The amount of fluid lying under the skin at the back of the baby's neck is measured and the result added to the measurement from the blood test.
• The combined test: a combination of the result of the NT scan and a blood test, offered early in pregnancy - Between 11 to13+6 weeks is offered in Hong Kong.
• The triple or quad test: a combination of a number of chemicals found in the blood, available after 15 weeks and before 20 weeks.

The result: Your risk or chance of having a baby with Down's syndrome is calculated as above or below NSC recommended cut-off limit. Your result should be available within one week and your doctor will tell you how you will get your results.

• High result: it is most likely that your baby does not have Down's syndrome. You will be offered an additional test to make an accurate diagnosis, such as CVS or Amniocentesis. More information about these tests are in "Diagnostic Tests for Chromosomal Abnormalities" section.
• Low result: it is important to be aware that none of the tests are 100% accurate, they detect between 70-90% of all cases.