Why should I consider having this blood test in my pregnancy?

As part of your prenatal care, you may be offered a blood test which can give helpful information about the possibility of certain problems in your baby's development (birth defects). This blood test, combined with your age, can provide an estimate of the risk that your baby has a particular birth defect such as a chromosomal abnormality or a neural tube defect. By itself, the blood test cannot identify any birth defect.

What is a chromosomal abnormality?

There are millions of cells in our bodies, each of which contains a complete copy of a person's genetic blueprint. This genetic material is packaged into 46 chromosomes. 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 occurs in about 1 out 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 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.

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 an individual risk estimate of the chances of having a baby with these types of birth defects. If your risk, based on the blood test, is higher than average for a particular birth defect it 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. The test is able to help detect around 60 (3 out of 5) of babies with Down syndrome and around 80% (4 out of 5) of those with neural tube defects.

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 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.

When should the test be done?

The blood test is done between 15 and 17 completed weeks since your last menstrual period. This blood test is optional and voluntary. 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.