Between the 15th and 20th weeks of pregnancy, expectant mothers usually are offered a blood test called the multiple marker test. Sometimes called a triple screen or a quad screen, depending on the number of things measured, it also might be done in combination with blood tests and an ultrasound in the first trimester.
For the multiple marker screening, a sample of blood is drawn from the mother to measure the levels of:
hCG (human chorionic gonadotropin), which is made by the placenta
estriol, which is made by the placenta and the fetus
alpha-fetoprotein (AFP), which is made by the fetus
Sometimes the level of inhibin-A, which is made by the placenta, also is measured. The levels of these substances can help doctors identify a fetus at risk for certain birth defects, including neural tube defects (like spina bifida) and some chromosomal abnormalities (like Down syndrome).
In determining the results of the test, doctors take into account factors such as:
the mother's age, weight, and ethnicity
whether she has diabetes
if she is having twins or other multiples
the gestational age of the fetus
These variables can affect the levels of the substances being measured and the interpretation of the test results, so the accuracy of this information is vital. If any of the information is incorrect, the screening results might be inaccurate.
If you have undergone the multiple marker test and received abnormal results, there is no need to worry yet. Just because the test is abnormal doesn't mean that your child has a birth defect or chromosomal abnormality. Rather, an abnormal screen indicates that the fetus should be evaluated further.
Usually, when a pregnant woman's results show high levels of AFP, pointing to a possible risk of spina bifida or another neural tube defect, her doctor will order a detailed ultrasound to examine the fetus, including the fetal skull and spine. In addition, an ultrasound can confirm the age of the fetus and whether the woman is carrying multiples. The doctor also may offer amniocentesis, which is the withdrawal of amniotic fluid from the uterus for further testing.
If a woman's multiple marker screen results reveal low levels of AFP and estriol and high levels of hCG and inhibin-A, she has an increased risk of having a baby with Down syndrome. The next step may be an ultrasound to confirm the baby's due date and to look for any obvious abnormalities. Unfortunately, ultrasound is not a very good test for detecting Down syndrome. For this reason, pregnant women are offered a follow-up test in order to get more information. This may be another screening test that looks at DNA from the fetus in a woman's blood, or amniocentesis so chromosome testing can be done on the fetal cells found in the amniotic fluid.
In general, remember that the multiple marker test is just a screen. It can identify many fetuses that are at risk for certain birth defects, but will not identify them all. A positive screen does not necessarily mean that there is a birth defect, but that there is a need for more evaluation.
If you have any questions or concerns about multiple marker testing, talk to your doctor or seek the advice of a genetic counselor.