Ultrasound scan
Since ultrasound provides an image of the baby in the womb, it detects structural abnormalities, particularly of the spine and head. Recently, however, it has been found to be useful in screening for Down’s syndrome and some other abnormalities of chromosome number. Several research studies have shown that the thickness of the ‘nuchal fold’ at the back of the baby’s neck is related to the risk of Down’s syndrome. An ultrasound scan at 10 to 13 weeks enables a measurement to be taken. This measurement then allows a risk factor to be calculated. The nuchal translucency scan is not widely available at present but it is becoming more so. It cannot be used to assess the risk of Down’s syndrome in twin pregnancies.
Alpha-fetoprotein (AFP) test
This test is performed at about 15 to 20 weeks to find out the level of alpha-fetoprotein (AFP) in your blood. This protein is made by your baby and passes into your blood during pregnancy. High levels are associated with spina bifida and so an ultrasound scan will then be offered to check for this. High levels may be seen in normal pregnancy and also in twin pregnancy. Low levels of AFP are associated with Down’s syndrome pregnancies. Ultrasound and amniocentesis will then be suggested to achieve a diagnosis.
Some hospitals routinely offer the AFP test to all women; others don’t, or restrict the test to older women since the risk of Down’s syndrome, and some other abnormalities of chromosome number, increases with age.
Serum screening
This is the term used for a test of the mother’s blood which screens for Down’s syndrome. It combines the AFP result (and so gives information about the risk of spina bifida) with the measurement of other blood chemicals to give the relative risk of having a baby with Down’s syndrome. There are various tests available (‘double test’, ‘triple plus test’, etc.) which differ slightly from each other, but they are all types of serum screening. They are not helpful in twin or other multiple pregnancies.
Some units give the result as ‘screen negative’ or ‘screen positive’. A negative result means that Down’s syndrome is unlikely. A positive result means that Down’s syndrome is more likely. An amniocentesis will be suggested to give more information. Other units give a numerical result, for example 1:250 risk of Down’s syndrome. You may like to compare this risk to that for your age (about 1:900 at 30) or to the risk of miscarriage with amniocentesis (about 1:100). Your doctor or midwife will explain the significance of the result to you. If the test is not available locally on the NHS, you may be able to obtain a test privately.
Amniocentesis
This test may be offered from 14 weeks of pregnancy:
- to women who have an AFP, serum screening or nuchal translucency scan result which indicates an increased risk of Down’s syndrome;
- when an ultrasound scan detects an abnormality which is associated with a genetic disorder;
- when a woman’s past or family history suggests that there may be a risk of her baby having a genetic disorder such as Down’s syndrome.
An ultrasound scan is performed to check the position of the baby and placenta. Whilst continuing to scan with the ultrasound probe, a fine needle is passed through the wall of the abdomen into the amniotic fluid which surrounds the baby. A small sample of this fluid is drawn off and sent to the laboratory for testing. Most women feel only mild discomfort.
Within the fluid are cells which contain the same chromosomes as the baby. Looking at these chromosomes is a complex process which is why the results take up to three weeks. This test will reveal your baby’s sex. Tell your doctor whether or not you want to know what it is. Some disorders such as haemophilia and muscular dystrophy are only found in boys (although girls may carry the disorder in their chromosomes and pass it on to their sons). Tell your doctor if these or other genetic disorders run in your family as it may then be important to know your baby’s sex.
Amniocentesis is associated with a 0.5–1% risk of miscarriage. At most, one test in a hundred 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. Remember that a normal result only reassures you about the number of chromosomes unless specific tests for disorders such as cystic fibrosis have been done.
Chorionic villus sampling (CVS)
This test is usually only available in large hospitals but smaller units are able to refer to these units if necessary. It tests for genetic disorders. It does not give information about spina bifida.
CVS can be carried out earlier than amniocentesis at around ten weeks but may carry a slightly higher risk of miscarriage, at about 1%. CVS before ten weeks has been associated with a slightly increased risk of limb deformities. Women at risk of having a child with an inherited disorder such as cystic fibrosis or muscular dystrophy may accept the increased risk of miscarriage in order to obtain an earlier diagnosis.
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 woman’s abdomen, or sometimes a fine tube through the vagina and cervix, into the womb. A tiny piece of the developing placenta, known as chorionic tissue, is withdrawn. Again, the chromosomes in the cells of this tissue are looked at. The results take up to two weeks.
If you feel the test would be helpful, talk over the matter carefully with your GP or midwife early in your pregnancy or before conception, as well as with your partner or a close friend. You can also contact your regional genetic centre direct.
Information provided by Health Promotion England.
