How is Down’s syndrome diagnosed?

All pregnant women in the UK are given the opportunity to undergo antenatal screening programmes, which can test for genetic conditions, birth defects and disorders such as blood and musculoskeletal conditions. Some women choose not to have the tests.

Antenatal screening tests can be carried out after the first ten weeks of pregnancy; the Down’s syndrome test is usually carried out between 11 and 13 weeks. The test for Down’s syndrome is known as the combined test because it uses both ultrasound scans and blood tests. The blood test is used to examine the make-up of the blood and monitor levels of proteins and hormones, while the ultrasound scan is designed to identify any structural and developmental abnormalities. During your pregnancy, you will have a few ultrasound scans to check the progress of your baby.

The ultrasound scan used to test for Down’s syndrome is known as a nuchal translucency scan; the scan is used to measure the distance between the nape of the baby’s neck and the spine. Babies with Down’s syndrome tend to collect a higher volume of fluid in their neck than babies without the condition so this measurement will help to determine if the baby has got Down’s syndrome.

If your blood or ultrasound scans show possible signs of Down’s syndrome, you will be advised to have further tests; these tests include CVS (chorionic villus sampling) and amniocentesis. The amniocentesis test involves taking a sample of the amniotic fluid, which surrounds the foetus in the womb; a fine needle is inserted through the wall of abdomen and a sample of fluid is collected using a syringe; the sample is then sent away to the laboratory. CVS involves testing a small sample of the placenta; this is done by inserting a fine needle into the womb or up through the vagina. Both tests carry risks but they are widely regarded as being very safe and can be very advantageous, especially if you are at a higher risk of having a baby with a congenital condition, genetic condition or birth defect.

Once you have had the baby, it will usually be obvious that the baby has got Down’s syndrome as the physical appearance differs to babies without the condition. If the doctor decides to do a diagnostic test after birth, this will involve a type of test known as a karotype; this test analyses the chromosomes in the blood.