Screening tests can reassure you that your baby has no detected abnormalities or give you time to prepare for the arrival of a baby with special needs. Testing can also provide valuable information for your care during pregnancy.
No test is 100 per cent certain. Some abnormalities may stay undetected.
If you do have a screening test and it suggests an increased chance of a chromosomal abnormality, you will be offered further tests which will give a more definitive diagnosis.
Testing for Down’s syndrome and other genetic anomalies
Blood testing for Down’s syndrome can be carried out between 11 and 13 weeks of pregnancy. It measures a number of blood chemicals associated with pregnancy to give your individual statistical chance of having a baby with Down’s syndrome. Blood testing on its own is not recommended for multiple pregnancies.
Getting the test results
Some maternity services give the result as ‘lower risk/screen negative’ or ‘higher risk/screen positive’.
If the test shows the risk of the baby having Down’s syndrome is lower than the recommended national cut-off, this is a low-risk result. A low-risk result means that you are less likely to have a baby with Down’s syndrome, but it does not mean there is no risk.
If the result shows the risk of the baby having Down’s syndrome is greater than the national cut-off, this is known as increased or higher risk. If your baby has an increased risk of having Down’s syndrome, you will be offered a further diagnostic test, but it does not mean that your baby definitely has the condition.
The diagnostic procedure you will be offered is called amniocentesis. Your midwife or doctor will explain the result to you and help you decide whether you want to have further tests. Amniocentesis can be offered from 15 weeks of pregnancy.
Using ultrasound as a guide, a fine needle is passed through the wall of the abdomen into the amniotic fluid that surrounds your baby. The fluid contains cells that have the same chromosomes as your baby. A small sample of this fluid is drawn off and sent for testing. Most women feel only mild discomfort.
The results are usually available with three working days, though it can take up to three weeks if all chromosomes have to be looked at. This test will reveal the sex of your baby, so tell your midwife or doctor if you do not want to know this yet.
Risks involved amniocentesis
Amniocentesis has a 0.5-1 per cent risk of miscarriage. At most, one test in 100 will result in pregnancy loss. When deciding whether to go ahead with this test, try to balance the risk of miscarriage against the value of the result to you.