During your antenatal care you can have a number of special tests including screening, and diagnostic tests to check for various complications or defects affecting the baby. These tests can be reassuring if they rule out something you are worried about, such as genetic defect like fibrosis, but they may also reveal a problem that makes you question whether you want to go on with your pregnancy.
The risk of having a Down’s syndrome baby can be assessed at around 11-14 weeks using a special ultrasound scan called a nuchal scan (‘’Nuchal’’ means neck). A shadow of a particular size and shape at the back of baby’s neck may indicate a higher risk of chromosome defects such as Down’s syndrome if it is thicker than normal in relation to the age of the mother. If the nuchal scan shows there may be Down’s, the mother will need amniocentesis to confirm the diagnosis. In fact, many centres now only carry out amniocentesis after a nuchal scan has been done.
Serum screening (Bart’s triple test)
Serum screening (Bart’s triple test) was developed by St. Bartholomew’s Hospital in London. A sample of the pregnant mother’s blood is taken between 14 and 20 weeks to measure the level of three substances- oestriol, human chorionic gonadotrophin, and alpha-fetoprotein. The results are assessed in relation to the mother’s age to predict the chance of her baby suffering from Down’s syndrome. If the chances seem high (more than one in 250), doctors will suggest you have amniocentesis. If you are not offered a Bart’s triple test you can ask to have it.
AFP test – Alpha- fetoprotein is found in varying amounts in your blood throughout pregnancy. Between 16 and 18 weeks the levels are usually low. If a blood test is done at this time and the levels are 2-3 times higher than the average of a sample group, it may show there’s a neurological problem. However, these problems are diagnosed with greater accuracy by ultrasound, and the AFP test is being used less frequently. If the test shows that the level of alpha-fetoprotein is abnormally low, it may suggest that the baby could have Down’s syndrome, and amniocentesis would be proposed. However, the nuchal scan has replaced AFP testing in many units as a screening test for Down’s syndrome.
Most maternity units now have a number of tests available to screen mothers for a variety of possible fetal abnormalities. These screening tests do not tell you for certain whether anything is wrong, but if a test shows that there may be a problem, you may have some more diagnostic tests to confirm or rule it out.