This blood test, combined with your age and other factors, can provide you and your doctor with an estimate of the risk that your baby has one of several possible birth defects.
Does the blood test identify all birth defects?
NO. By itself, the blood test cannot identify any birth defect. However, the blood test can tell you and your doctor that there may be a greater possibility that your baby has a particular birth defect such as a chromosomal abnormality or a neural tube defect.
What is a chromosomal abnormality?
Chromosomes are tiny "packets" of inherited instructions (genes) which guide our development. There are normally 46 chromosomes in every cell in our body. Chromosomal abnormalities are changes in the number or structure of the chromosomes. These may result in problems in the baby's development, such as Down syndrome, which is a well-known chromosomal abnormality.
Down syndrome occurs in about 1 of every 700 babies. The chances that a woman will have a Down syndrome baby, or a baby which has a different type of chromosomal abnormality, increases with her age. If you are over 35 years old, you may want to consider genetic counselling to discuss your options for prenatal diagnosis.
What are neural tube defects?
The most common types of neural tube defects are anencephaly and spina bifida. Anencephaly is the most serious as the baby's brain is not developed properly and the babies do not survive. Spina bifida refers to an opening on the baby's spine which exposes the spinal cord and can cause paralysis and other problems.
In Australia, neural tube defects occur in about 1 in 800 babies. The risk for having a baby with a type of neural tube defect may be increased if :
- there is a close family history of neural tube defects
- if a woman has insulin-dependent diabetes
- if a woman is taking specific medicine to control seizures (epilepsy).
Genetic counselling is recommended in these cases.
What will the results of the blood test tell me?
Every woman has a risk of having a baby with a chromosomal abnormality or a neural tube defect. This blood test will give you a new risk estimate of the chances of having a baby with these types of birth defects.
Your risk, based on the blood test, may be higher than average for a particular birth defect. This does not mean that there is a problem in your unborn baby. The result indicates the need to consider having further tests called prenatal diagnosis such as ultrasound or an amniocentesis. You should discuss your options with your doctor. Written information is also available.
Most often, the blood test will give you a "low risk" estimate. This means that your chance of having a baby with a chromosomal abnormality or a neural tube defect is reduced. However, these problems, or any other birth defect, could occur in a baby even though the blood test indicated a low risk.
Remember, if your risk estimate is higher than average, it means that further testing should be considered. Such testing most often shows that the baby's development is normal.
When should the test be done?
The blood test is done between 15 and 17 completed weeks since your last menstrual period.
Does everyone have this blood test?
No. This blood test is optional and voluntary. Keep in mind that the finding of a "low risk" result is meant to be reassuring but it does not exclude birth defects including Down syndrome and neural tube defects. Also, if your result shows that your risk estimate is now "higher than average", you may be offered additional testing which could identify a problem in your baby's development. Some couples would prefer not to be faced with these possibilities during the pregnancy and will choose not to have the blood test.
It is important that you consider all aspects of this blood test before agreeing to have it done. If you would like more information about this blood test, please discuss it with your doctor, or call one of the centres listed on the back of this pamphlet which specialise in prenatal diagnosis and related genetic counselling.
Find a Prenatal Diagnosis Centre in Australia