TESTS TO DETECT ABNORMALITIES
You will be offered screening tests that can detect structural abnormalities like spina bifida, which is a defect in the development of the spine, or some chromosomal disorders like Down’s syndrome, which is caused by an abnormal number of chromosomes. Different maternity services may use different tests, but all tests carried out in the NHS will meet national standards. Discuss the tests and what they mean with your midwife. Screening tests can:
• reassure you that your baby has no detected structural abnormalities
• provide you with an opportunity to see your baby during the scan
• give you time to prepare for the arrival of a baby with special needs
• allow you to consider the termination of an affected baby. Tests can also provide valuable information for your care during the pregnancy. However, no test can guarantee that your baby will be born without an abnormality. No test is 100% accurate and some abnormalities may remain undetected
Deciding whether to have tests
You may not want to have a screening test if you think that you would continue your pregnancy whatever the results. If you do have a screening test and it suggests an increased chance of a chromosomal abnormality, you will be offered diagnostic tests, which will give a more definite diagnosis. These diagnostic tests carry a small risk of miscarriage, so you may decide not to have them. Discussing the issues with your partner, midwife, doctor and friends may help you in deciding what is right for you.
TESTS FOR DOWN’S SYNDROME AND OTHER GENETIC DISORDERS
These tests are offered to all pregnant women. There are different ways of carrying out screening.
Combined screening Combined screening involves a blood test and an ultrasound scan. All women should have dating and nuchal translucency scans between eight and 14 weeks. These scans may be combined. The blood test measures two pregnancy-associated blood chemicals. At the scan, the radiographer measures the thickness of the nuchal translucency at the back of your baby’s neck. This information is used to calculate your individual statistical chance of having a baby with Down’s syndrome. This nuchal translucency scan can be used in multiple pregnancies
Serum screening is a blood test that screens for Down’s syndrome, usually at about 16 weeks into your pregnancy. It measures three or four pregnancy-associated blood chemicals to give your individual statistical chance of having a baby with Down’s syndrome. Serum screening on its own is not recommended for twin and other multiple pregnancies.
Some maternity services give the result as ‘lower risk/screen negative’ or ‘higher risk/screen positive.
If the screening test shows the risk of the baby having Down’s syndrome is lower than the recommended national cut-off, this is known as having a ‘low-risk’ result. A low-risk result means that you are at a low-risk of having 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 recommended national cut-off, this is known as an ‘increased risk’ or ‘higher risk’ result. An increased risk means you will be offered diagnostic test but it does not mean that your baby definitely has the condition. The diagnostic procedure you will be offered is either chorionic villus sampling (CVS) (see next page) or amniocentesis to give you a definite answer about Down’s syndrome. Your midwife or doctor will explain the result to you and help you decided whether you want to have further tests
DIAGNOSTIC TESTS FOR DOWN’S SYNDROME AND OTHER GENETIC DISORDERS
These tests will give you a definite diagnosis of Down’s syndrome and sometimes other abnormalities. Your midwife or doctor will explain what is involved and you will usually be offered counseling.
Chorionic villus sampling (CVS)
CVS can be carried out at around 11 weeks. It can give you an earlier diagnosis if you are at risk of having a child with an inherited disorder, such as cystic fibrosis, sickle cell disorder, thalassemia or muscular dystrophy.
The test takes 10 to 20 minutes and may be a little uncomfortable. Using ultrasound as a guide, a fine needle is passed through the abdomen into the uterus.
Sometimes a fine tube is passed through the vagina and cervix into the uterus instead. A tiny piece of the developing placenta, known as chorionic tissue, is taken. The chromosomes in the cells of this tissue are examined. As with amniocentesis, a rapid result can be obtained, but if all the chromosomes are going to be checked the results may take up to two weeks. The risks
CVS has a 1–2% risk of miscarriage. This is slightly higher than amniocentesis.
IF A TEST DETECTS AN ABNORMALITY
It is always difficult when you are told there is something wrong with your baby, especially if you are faced with a painful decision about the future of your pregnancy. Your midwife or doctor will make sure you see the appropriate health professionals to help you get all the information and support you need so you can make the choices that are right for you and your family
Amniocentesis can be offered from 15 weeks of pregnancy if:
• you have a positive or higher risk Down’s syndrome screening result
• an ultrasound scan detects an abnormality that is associated with a genetic disorder
•your past history or family history suggests that there may be a risk of your baby having a genetic or chromosomal disorder such as Down’s syndrome, sickle cell disorder or thalassemia.
What happens Using ultrasound as a guide, a fine needle is passed through the wall of the abdomen into the amniotic fluid that surrounds your baby. Within the fluid are cells that contain the same chromosomes as your baby. A small sample of this fluid is drawn off and sent to a laboratory for testing. Most women feel only mild discomfort. Usually, the fluid will be tested for Down’s syndrome and other serious syndromes. The results should be available within three working days. If all the chromosomes have to be looked at, it can take up to three weeks. This test will reveal your baby’s sex, so tell your midwife or doctor whether, at this stage, you want to know if your baby is a boy or a girl.
Amniocentesis has a 0.5–1% risk of miscarriage. At most, one test in 100 will result in pregnancy loss. When deciding whether or not to go ahead with this test, try to balance the risk of miscarriage against the value of the result to you.