Maternal Screening
At a Glance
Why Get Tested?
For a pregnant mother to assess the chance of her baby being born with an abnormality such as Down’s syndrome or spina bifida.
When to Get Tested?
Screening is available for Down’s syndrome in some centres from about the 9th week of pregnancy. In most centres women will be offered screening at about 16 weeks, although it can be carried out until about 21 weeks of pregnancy.
Sample Required?
A blood sample taken from a vein in the arm. The results from the laboratory may be combined with the results from an ultrasound scan to improve the overall effectiveness of the screening test.
The Test Sample
What is being tested?
The screening tests involve the measurement of different proteins and hormones found in mother’s blood.
Several different biochemical substances are measured in the blood. Which substances are measured depends on the time during pregnancy that the sample was taken. Different substances are better indicators of a problem at different times during pregnancy. The combinations of tests may be known by different names depending usually on how many tests are measured. For example, the triple test combines the results of three different substances.
The following screening tests may be performed:
- alpha fetoprotein (AFP) is a protein produced by the developing baby. AFP moves from the baby across the placenta and amniotic membranes into the mother and appears in the maternal blood. The level in the mother’s blood tends to rise throughout the first half of pregnancy. It is usually measured at about 16 weeks of pregnancy.
- human chorionic gonadotrophin (hCG) is a hormone produced by the placenta. Levels in mother’s blood rise early in pregnancy and then fall later on.
- unconjugated oestriol (uE3) is a hormone produced by the baby as well as by the placenta. It is usually measured if screening is performed at about 16 weeks gestation.
- pregnancy associated plasma protein A (PAPP-A) is a protein produced by the placenta. The level in mother’s blood rises throughout the first half of pregnancy and it is usually measured if screening is performed early in pregnancy.
How is the sample collected for testing?
Blood is taken from a vein in the arm and all the necessary tests can be measured from the same tube of blood.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
The Test
How is it used?
The triple test is used as a primary screening test to assess the chance of the pregnancy being affected by Down’s syndrome. The results from all of the biochemical tests are used together to estimate the possibility of a Down’s syndrome affected baby. A raised level of AFP by itself might indicate the possibility of a neural tube defect.
The following screening tests may be performed:
- The maternal AFP level tends to be lower than usual in pregnancies affected by Down’s syndrome. A baby with an open neural tube defect has an opening in the spine, head or abdominal wall that allows higher than usual amounts of AFP to pass into the mother’s blood.
- The maternal hCG level tends to be higher than usual in pregnancies affected by Down’s syndrome.
- In pregnancies affected by Down’s syndrome the oestriol level tends to be lower than usual when measured at about 16 weeks.
- Values of PAPP-A tend to be lower in Down’s syndrome affected pregnancies when measured before 13 weeks.
Calculation of the likelihood of a Down’s syndrome affected pregnancy
A detailed mathematical calculation is performed, using the results of the various biochemical screening tests, together with details of mother’s age and weight, to calculate the chance of the baby being affected by Down’s syndrome, or another chromosomal abnormality such as Edward’s syndrome. Other test results may be included in this calculation such as the nuchal translucency measurement from an ultrasound scan. Additional information about family origin and smoking history may be included in the calculation to improve its efficiency. If the chance is greater than a certain cut-off value, further tests may be offered.
When is it requested?
The screening tests may be performed as early as 9 or 10 weeks of pregnancy in some centres or around 16 weeks in others. Screening can be performed up until about 21 weeks of pregnancy.
What does the test result mean?
The interpretation of the screening test result should be provided by the midwife, a genetic counsellor or doctor. They will be able to explain the meaning of the result and offer the choices available to the mother. If the result appears to be in the ‘higher risk’ category, more definitive tests are needed to confirm a diagnosis. For possible chromosome abnormalities, an amniocentesis or chorionic villus sample will usually be needed. Other disorders may be detectable using a detailed ultrasound scan.
Is there anything else I should know?
It is important to remember that the tests described here are screening tests and not all Down’s syndrome affected pregnancies will give results in the ‘higher risk’ category. The screening results are a guide to those women who might benefit from further more invasive tests. Only a small proportion of women who are in the ‘higher risk’ category for Down’s syndrome, or have a high AFP result, will have an affected baby.
The screening results are very dependant on an accurate age for the baby and this is why a dating ultrasound scan is recommended prior to screening. If the gestational age of the baby is not correct, the screening result may be falsely high or low. An early ultrasound scan will show the possibility of twins or triplets and these will result in high maternal AFP levels.
More definitive tests are needed to diagnose a chromosome abnormality or a neural tube defect. To diagnose Down’s syndrome, an amniocentesis or chorionic villus sampling procedure must be performed. A small amount of fluid surrounding the baby, or a small piece of the developing placenta, is taken using a very fine needle. It is then possible to use laboratory techniques on one of these samples to look at the chromosomes of the baby and detect possible problems. There is a small risk of infection or miscarriage associated with these invasive procedures and therefore careful consideration should be given before having them performed.
The likelihood of carrying a baby with Down’s syndrome increases with mother’s age. Over the age of 35, about 40% of women will be in the ‘higher risk’ group after screening, just because their age-related risk is so high. However, it is important to stress that most women in the ‘higher risk’ group will NOT have an affected baby.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
