Also known as: Down syndrome screening, triple test
Name: Prenatal screening
Related tests: hCG
Why is the test done?
To assess the risk of fetal abnormalities such as Down syndrome.
When is it done?
In women between the 16th and 21st week of pregnancy.
What samples are required?
A blood sample taken from a vein.
What is being tested?
The test measures three or four substances in the blood: AFP, hCG, unconjugated oestriol and inhibin A. Inhibin A is a new marker recently added to the screening, increasing both its sensitivity and its specificity.
• During fetal development, AFP levels in fetal blood and amniotic fluid rise until the 12th week and then gradually fall until birth. AFP is a fetal protein that crosses the placenta and can be detected in the mother’s blood.
• hCG is a hormone produced by the placenta. Levels in the mother’s blood rise during the first trimester of pregnancy and then fall during the second and third trimesters.
• Unconjugated oestriol is produced by fetal metabolism in the liver, adrenal glands and placenta. Some oestriol crosses the placenta and can be measured in the mother’s blood.
• Inhibin A is a hormone produced by the placenta; the molecule is a dimer, i.e. it is made up of two subunits, and is therefore also called DIA or dimeric inhibin A. Levels in the mother’s blood fall slowly from the 14th to the 17th week of pregnancy and then rise again.
A baby with a neural tube defect has an opening to the outside at the level of the spine, skull or abdominal wall. This condition is frequently associated with high AFP levels in the mother’s blood; the other markers are not useful in assessing the risk of this condition.
In pregnant women whose fetus has the chromosomal defect characteristic of Down syndrome, AFP and unconjugated oestriol levels tend to be low, while hCG and inhibin A are high.
How is the sample collected for testing?
A venous blood sample is taken. All four tests can be carried out on the same tube of blood.
How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?
How is it used?
The triple test is an early screening test in pregnancy. It indicates the risk of having a fetus with abnormalities such as Down syndrome or neural tube defects.
• If AFP is high, the fetus has a greater risk of a neural tube defect such as spina bifida or anencephaly.
• The mathematical calculation takes into account the levels of AFP, hCG, unconjugated oestriol and sometimes inhibin A when assessing the risk of chromosomal abnormalities. The mother’s characteristics, such as age, weight, ethnicity and any diabetes, are also taken into account. The resulting risk value is compared with a cut-off: if the value is higher than the cut-off, the test is considered positive and the risk increased.
When is it requested?
The test is requested between the 15th and 20th week of pregnancy.
What does the test result mean?
The test should be interpreted by a genetic counsellor or doctor who can explain what the result means and advise on choices about the pregnancy. If the test is positive, further investigations, such as an ultrasound scan and possibly amniocentesis, are needed to confirm any diagnosis. This test helps the woman and her doctor make decisions about managing the pregnancy.
Is there anything else I should know?
This is a screening test. Not all fetal abnormalities are detected by the test. Of all the women who test positive, only a small number will go on to have a baby with a neural tube defect or a chromosomal abnormality. Further investigations are always needed to diagnose these abnormalities in the fetus. Multiple pregnancies (e.g. twins or triplets) can cause high AFP levels. The test result also depends on an accurate determination of the fetus’s gestational age: if this is not accurate, the result may be falsely positive or negative.
In general, the risk of having a fetus with Down syndrome increases with the mother’s age. About 40% of women over 35 may test positive because they have a higher age-related risk.
1. What is Down syndrome?
2. What is a neural tube defect?
3. What can be done during pregnancy to prevent a neural tube defect?
1. What is Down syndrome?
Down syndrome is a chromosomal abnormality also known as trisomy 21. Affected people carry an extra copy of part or all of chromosome 21. Many affected children have delayed growth and impaired intellectual development. The risk of Down syndrome increases with the mother’s age, especially if she is over 40.
2. What is a neural tube defect?
A neural tube defect is a serious birth defect in which the brain, the spinal cord or the structures covering them have not developed completely. There are three types of defect:
• Anencephaly: incomplete development of the brain and skull.
• Encephalocele: herniation of brain tissue through an opening in the skull.
• Spina bifida: the most common defect, in which the spine does not close completely during the early stages of pregnancy.
3. What can be done during pregnancy to prevent a neural tube defect?
All pregnant women are recommended to take 0.4 mg of folic acid a day.