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