screening tests

Screening tests are available to all pregnant women. They are non-invasive and carry no risk of miscarriage. Screening tests can involve a maternal serum screening and/or an ultrasound.

Ultrasound involves using sound waves to take a picture of the foetus. Gel is spread over the lower abdomen of the pregnant woman and then a transducer (a metal instrument) is moved over the skin. The sound waves are analysed by a computer to give a scan that can be viewed on a television. This process is painless and there are no known hazards. During an ultrasound, a foetus can be detected as having Downs Syndrome if “nuchal translucency” is seen – a thickening of skin over the neck.

There are two ideal times for an ultrasound:
1st Trimester ultrasound – nuchal ultrasound at 11-13 weeks
2nd Trimester ultrasound – (also called foetal anomaly scan) 18-20 weeks, identifies structural problems, but does not detect all abnormalities

Maternal serum screening is a blood test which can identify pregnant women who are at an “increased risk” of having a child with Downs Syndrome (this test can also be used for neural tube defects and Trisomy 18). A blood sample is taken from the mother and the amounts of hormone and foetal protein are measured. Mothers with foetuses that have Downs Syndrome have abnormal amounts of these proteins compared to mothers with unaffected pregnancies.

Two maternal screening tests are available in Victoria – the mother can have one or the other, but not both:
1st Trimester combined screening: a blood test done at 9-12 weeks (best at 10 weeks) combined with a nuchal translucency ultrasound scan at 11-13 weeks
2nd Trimester maternal serum screening: a blood test at 14-20 weeks

Maternal serum screening tests can detect 60% of foetuses with Downs Syndrome; however, when combined with an ultrasound at 11-13 weeks into the pregnancy, the sensitivity increases to about 90% with a 5% false-positive rate. Nevertheless, positive tests from screening tests are not definitive enough to diagnose Downs Syndrome – further procedures/tests will be required such as amniocentesis.

Life expectancy of for people with Downs Syndrome has improved in the last 40 years (from an average of 16years to 57.8 years for women and 61.1 years for men). Current estimates for life expectancy of a person with Downs Syndrome vary from 30-40 years, although children who survive infancy are expected to live longer than this (2006).