Prenatal diagnostic tests, conducted during pregnancy, may determine whether the baby has a particular problem. If you are concerned that your child may have a serious disorder, ask your doctor if tests can be conducted.
It is important to remember that prenatal tests do not guarantee a perfect baby. They can only test for specific problems. Most prenatal tests are only recommended if you fall into a high-risk category. That is, if you are over 35 years old, or you have a family history of genetic disorders.
Ultrasound
Besides revealing the baby's age and whether or not you are carrying twins, ultrasound can also detect some physical abnormalities. The doctor rubs a jelly-like substance onto your abdomen before pressing an instrument like a microphone against your skin.
Sound waves pass into the uterus, bouncing off the baby. The sound waves are translated by a computer into a picture on a television screen. The test is believed to be harmless to you and your baby.
Chorionic Villus Sampling
This test must be done in the 10th to 12th week after the first day of your last period. If you think you need this test, it is vital to see a doctor as soon as you think you might be pregnant.
By testing cells from tissues surrounding the baby, the Chorionic Villus Sampling (or 'CVS') can detect chromosomal abnormalities such as Down syndrome and inherited abnormalities such as cystic fibrosis and thalassaemia. Results are usually available in two to three weeks.
CVS uses ultrasound to locate the tissue needed for testing. A fine tube then goes through the cervix into the uterus to remove a tiny amount of tissue (it is a bit like having a Pap smear taken). Sometimes the tissue is removed by passing a needle through the abdomen. You don't need an anaesthetic.
When the test is carried out by a doctor experienced in CVS, the risk of miscarriage from the test is about one in 50 pregnancies (including miscarriages in those women who may have miscarried without the test).
Nuchal Translucency Testing
This is a screening test in the form of an ultrasound scan performed at 11 to 13 weeks. It measures the thickness at the back of the neck of the foetus. This thickness is called nuchal translucency. A relationship exists between it and the risk of having a baby with Down syndrome.
A computer program can generate a new risk ratio (e.g. 1: 800) from the combination of the age risk and the nuchal translucency risk. This new risk is used to determine if the woman should go on to have invasive testing, which is the only way to know if the baby does or doesn't have Down syndrome.
Amniocentesis
Amniocentesis is done in the 14th to 18th week of pregnancy. It can detect chromosomal abnormalities, inherited disorders and neural tube defects such as spina bifida (where the baby's spinal cord doesn't develop properly). Results take about three to four weeks.
For amniocentesis, an ultrasound is used to locate the amniotic fluid. This is the fluid inside the amniotic sac, the balloon-like "bag of waters" in which the baby floats. A hollow needle then goes into the abdomen and draws out a small sample of fluid.
When amniocentesis is carried out by an experienced doctor, the risk of miscarriage from the test is one in 100 pregnancies or even lower (including miscarriages in those women who may have miscarried without the test).
Alpha Fetoprotein Test (AFP) or Maternal Serum Test
This blood test is done in the 16th or 18th week of pregnancy and can provide you and your doctor with an estimate of the risk (or chance) that your baby has certain birth defects. It checks the level of hormone called alpha fetoprotein (AFP) in your blood. Too much AFP in the blood occurs in three in 100 women. It may mean the baby has a neural tube defect, such as spina bifida or anencephaly (where the skull and brain don't develop properly).
However, if there is a high level of AFP in your blood, don't panic. There can be other causes. It could mean you are having twins, for instance, or that your pregnancy is further along than you thought. It doesn't necessarily mean the baby is affected in any way.
If this happens to you, your doctor may suggest an ultrasound examination to check whether there is a neural tube defect.
If the level of alpha fetoprotein (AFP) in the blood is found to be very low (this happens in less than five in 100 pregnancies), the levels of two other hormones in the blood may also be measured. This is called a triple screening, and is done through your doctor or hospital.
By looking at the levels of these three hormones and considering other factors such as your age, weight and how many weeks pregnant you are, triple screening can assess your risk of having a baby with Down syndrome or certain other chromosomal abnormalities. If the result shows a high risk for the condition, you can then decide whether to have an amniocentesis, to determine for certain whether your unborn baby has Down syndrome.
Chromosomal Abnormalities
Chromosomes are tiny structures present in every cell of your body. Each chromosome contains thousands of genes (the "instructions" that decide how each of us will look and develop). Birth abnormalities may result if a baby has too many or too few chromosomes. The most well-known chromosomal disorder is Down syndrome which occurs when a baby has an extra chromosome in body cells.
What If A Test Shows An Abnormality?
The chances are it won't. However, if an abnormality is diagnosed, parents will be given the necessary information to make a choice about whether to continue with the pregnancy. For parents who are unsure about their decision, support and counselling are available.
With the exception of the information about Nuchal Translucency Testing, which was provided by Dr Brian Peat this information was reproduced with permission from Pregnancy Care, a publication of the NSW Health Department. Dr Brian Peat is an obstetrician at Sydney's King George V hospital, specialising in the prevention of complications in high risk pregnancies. He is also involved in educating medical students, midwives and medical graduates.