Second trimester maternal screening

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.

Second trimester biochemical screening is usually performed between 14 and 20 weeks of pregnancy.

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.

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.

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.