Different types of screening tests are available during your pregnancy, one of the common tests is an optional screening test for Down syndrome.
A screening test will give you an indication of your risk of carrying a baby with chromosome abnormalities such as Down Syndrome. It does not give a definite answer, but it does tell us which babies have an increased risk of having Down syndrome. The results may then help you in your decision about further diagnostic testing during pregnancy. Screening tests are simple and non invasive but do have out-of-pocket expenses attached for the ultrasound and blood test.
What is Down syndrome?
Cells of the body usually contain 23 pairs of chromosomes, making a total of 46. In people with Down syndrome, all or some of the cells in their body contain 47 chromosomes, where there is an extra copy of chromosome 21. It is this extra genetic material that results in the intellectual and physical characteristics associated with Down syndrome.
Why Do We Screen for Down syndrome?
Down syndrome is the most common chromosomal cause of intellectual disability. It can occur in any pregnancy regardless of the woman’s age. However, the chance of having a baby with Down syndrome increases with the mother’s age.
What is The First Trimester Combined Screening test?
The first trimester combined screening test is a two part test, which involves a blood test and an ultrasound in your first trimester of pregnancy. During the blood test, a hormone (Beta human chorionic gonadotrophin) and a protein (pregnancy associated plasma protein A) are measured. At Sydney Ultrasound for Women, we offer the first trimester ultrasound between the 12th and 13th week of pregnancy. During this scan we measure the gestational age of the fetus by measuring from head to bottom to get the crown-rump length (CRL) and the nuchal translucency.
By combining your age, the results of your ultrasound and the results of two proteins in your blood we can determine your risk of having a baby with Down syndrome and other chromosomal abnormalities. The accuracy of detecting Down syndrome is approximately 85-90% by utilising this screening test.
What Results Will I Receive From This Test?
If the likelihood of having an abnormality is higher than a certain cut-off value, the screening test results will be classified as an increased risk.
If the risk of an abnormality is lower than that cut-off value, the screening test results will be classified as low risk.
Once the screening results become available (usually within 48 hours), you will be informed by one of our genetic counsellors of the results, you will also be informed of the numerical chances that an abnormality is present.
What Does My Result Mean?
It is important to understand that a low risk screening result does not rule out an abnormality. Similarly, an increased risk result does not indicate that an abnormality is present. Instead, an increased risk result may prompt you to pursue further tests, which include chorionic villous sampling (CVS) or amniocentesis.
What if I Missed the First Trimester Combined Screening Test?
If your gestation is greater than 13 weeks 6 days of pregnancy, other screening tests are available in your second trimester of pregnancy. You may wish to discuss these tests further with your referring doctor or genetic counsellor.