Second trimester maternal screening
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 screening tests involve the measurement of different proteins and hormones found in mother’s blood. In addition, the mother is asked to provide additional information about herself that is used to improve the accuracy of the screening test, such as her age, weight, ethnicity, smoking status and whether she has diabetes. The following blood tests may be performed as part of second trimester screening:
- 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. Levels in the mother’s blood tend to rise throughout the first half of pregnancy.
- Total of free beta human chorionic gonadotrophin (hCG) are hormones 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. Levels gradually increase through the first and second trimester of pregnancy.
- Inhibin-A is a hormone produced by the placenta. Levels rise during the first trimester of pregnancy and are generally constant in the second trimester.
The tests may be known by different names depending on how many tests are performed. For example, the triple test combines the results of three tests (AFP, hCG and uE3) while the quadruple test combines the results of four tests (AFP, hCG, uE3 and Inhibin-A).
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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
Second trimester maternal screening is used primarily to assess the risk of a pregnancy being affected by Down’s syndrome, although some laboratories also provide a risk for Edward’s syndrome. The results from all of the biochemical tests are combined to estimate the risk of a pregnancy being affected by Down’s syndrome and/or Edward’s syndrome. In addition, a raised level of AFP by itself might indicate the possibility of a neural tube defect, although screening for neural tube defects is now predominantly carried out by ultrasound scanning.
Pregnancies affected by Down’s syndrome, Edward’s syndrome or a neural tube defects tend to give predictable changes in each of the test results:
- AFP levels tend to be lower than usual in pregnancies affected by Down’s syndrome and they tend to be higher in pregnancies affected by neural tube defects.
- hCG levels tends to be higher than usual in pregnancies affected by Down’s syndrome.
- uE3 levels tend to be lower in pregnancies affected by Down’s syndrome.
- Inhibin-A levels tend to be higher in pregnancies affected by Down’s syndrome.
Levels of all markers tend to be lower in pregnancies affected by Edward’s syndrome.
Calculation of the likelihood of a Down’s syndrome affected pregnancy
A mathematical calculation using the results obtained from the blood tests and other factors, such as the mother’s age, is used to determine a numeric risk of the pregnancy being affected by Down’s syndrome. This risk is compared with an established cut-off. If the risk is higher than the cut-off value (e.g. risk of 1 in 150 or higher), then it is considered that the woman may be at increased risk for having a baby with Down’s Syndrome.
When is it requested?
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 screening result does suggest a high risk of Down’s syndrome then a more definitive test is offered to the mother that can confirm the diagnosis. In the second trimester this is usually involves a procedure called amniocentesis, which carries a small risk of miscarriage (approximately 1 in 100 ) and a very small risk of injury to the fetus.
If the screening test suggests a high risk of a neural tube defect (e.g. a high AFP result) then detailed ultrasound scans are usually required.
The charity Antenatal Results and Choices (ARC) provide further information and support for parents undergoing antenatal screening.
Is there anything else I should know?
As with all screening tests, biochemical testing for Down’s syndrome affected pregnancies is not perfect. If all four of the above tests are performed (i.e. a Quadrulple test) then approximately 80% of Down’s syndrome cases can be correctly identified. However, the test will incorrectly identify approximately 4% of healthy pregnancies as being high risk for Down’s syndrome (i.e. false positives). This means that some of the pregnancies that the test classifies as ‘high risk’ may actually be normal, healthy pregnancies.
The overall test result is very dependent on accurate determination of the gestational age of the fetus. If the gestational age of the fetus has not been accurately determined, the results may be either falsely high or low. It is also important that all the necessary details from the mother’s clinical history have been recorded accurately as this can have a significant impact on the accuracy of the test outcome.
In twin pregnancies, calculation of the risk of Down syndrome is more complicated. A "pseudorisk" can be calculated by comparing results to normal results in other twin pregnancies but the detection rate is lower (approximately 60-65%). For higher gestation pregnancies (e.g. triplets), risk cannot be calculated using these tests.
What is Down's Syndrome?
Down's Syndrome is produced by having an abnormal number of chromosomes and is sometimes called Trisomy 21. Affected people usually have an extra copy of some or all of chromosome 21. Most affected children have some retardation of growth and development. The risk of Down's syndrome increases with the mother's age, especially in women over 40 years old. For more information, visit the Down's Syndrome Association UK website.
Edward’s syndrome is caused when babies have an extra copy of chromosome 18. This condition is more severe than Down’s syndrome and sadly most babies with Edwards syndrome are miscarried or stillborn. For more information visit NHS Choices website.
What is a neural tube defect?
Neural tube defects are serious birth defects: the brain, spinal cord, or their coverings do not develop completely. There are 3 kinds of neural tube defects:
- Anencephaly: incomplete development of the brain and the skull.
- Encephalocoele: a hole in the skull through which brain tissue protrudes.
- Spina bifida: the most common neural tube defect, in which the spine does not close properly during early pregnancy. (For more information on spina bifida, visit the Association for Spina Bifida and Hydrocephalus or the Scottish Spina Bifida Association.)
What can I do during pregnancy to prevent formation of a neural tube defect?