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Maternal Serum Screening Test

A blood test in early pregnancy to aid the detection of Down’s syndrome and neural defects (anencephaly and spina bifida.)

What is the maternal serum-screening test?

It is a blood test to measure the levels of three substances in a pregnant woman’s blood. The substance is alpha-fetoprotein (AFP), uncnjugated oestriol (uE3) and human chronic gonadotrophin (hCG). The test is usually carried out about 16 weeks of pregnancy. The levels of the three substances are used in combination with the woman’s age to estimate the risk of Down Syndrome. The level of AFP alone is used to determine if there is an increased risk of a neural tube defect.

The maternal serum-screening test identifies women with an increased risk of Down’s syndrome or neural tube defects so they can decide whether or not to have the necessary diagnostic test. The diagnosis of Down’s syndrome requires an amniocentesis (a sample procedure to sample the fluid around the baby) and the diagnosis of neural tube defects is made by caring out a detailed ultrasound scan as well as an amniocentesis.

The result of the maternal serum-screening test is reported as either “screen-negative” or “screen-positive.”

What does a “screen- positive” test result mean?

A woman may be “screen-positive” for two reasons:

  1. Screen-positive because of an increased risk of neural tube defects. This means that the risk of having a pregnancy with a neural tube defects is increased due to the fact that the level of AFP in the mother’s blood is more than two and half times the normal value.
  2. Screen-positive because of an increased risk of Down syndrome. This means that the risk of giving birth to a baby with Down’s syndrome, taking into account the woman’s age as well as her blood levels of AFP, uE3 and hCG, is 1 in 385 or more

Most women with “screen-positive” best results will normally have healthy babies. A screen-positive test result does not mean that there is an abnormality. It only means that there is an increased risk and further tests are indicated.

What are the further tests?

The further tests are:

  1. An ultrasound “dating” scan to check how far the pregnancy is advanced (only one has already been done.) AFP, uE3 and hCG levels in the blood vary as the pregnancy goes along, so that levels which may be high or low for an 18-week pregnancy would be normal for 15 week pregnancy. If the ultrasound-dating scan shows that the pregnancy is not as far (or is further) advanced than would be expected by date (time from the date of the last menstrual period), then the screening test will need to be re-interpreted and may no longer be regarded as positive
  2. Diagnostic test (amniocentesis and a detailed ultrasound scan) if the screening test is still positive after the “dating” scan

What does a “screen-negative” test result mean?

If the risk of giving birth to a baby with Down Syndrome is found to be less than 1 in 385, and the AFP level is not high, then the screening result is called “syndrome” negative. More than 9 out of 10 women will have negative results.

Does a “screen-negative” test completely Down Syndrome and neural defects?

No, some cases are not detected. The screening test will detect

  1. 2 out of 3 cases of Down Syndrome
  2. Almost all cases of anencephaly
  3. Approximately 4 out of 5 cases of babies with open spina but closed spina bifida is not normally detected by AFP screening

What happens if the diagnostic test shows that there is an abnormality?

The results will be discussed with the woman concerned and information will be provided about the type of defects that has been found and how it may affect the baby. The possibility of termination of pregnancy will be discussed.

Please note:

The test must be done after 15 weeks of pregnancy and preferably by 18 weeks. Screening is most accurate when done between 16-18 weeks.

If you have a twin pregnancy or have insulin dependent diabetes, this must be noted in the information submitted with the test.

If an amniocentesis has been attempted in this pregnancy, the test cannot be interpreted.