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A birth defect
is a physical problem that a baby has at birth. Most babies
– 97% – are born without major birth defects. Some birth
defects may be inherited from the parents. Others may result from
a virus or exposure to a substance during pregnancy, such as alcohol
or drugs. Most often, the reason for the defect is unknown.
Most babies with birth defects are born to women and men with
no risk factors.
There are tests available that give information about a pregnant
woman's risk of having a baby with certain birth defects, such
as Down syndrome
or spina bifida. One of these is a blood test called a maternal
serum screening test. It tests for products from the pregnancy
that are also in the woman's blood (serum).
A screening test is a test that is performed when there are no
symptoms or known risk factors present. It is not a diagnostic
test. A screening test can only show whether you may be at risk
of having a baby with a certain birth defect. A diagnostic test
can usually show whether your baby has the birth defect or not.
If your screening test shows a higher-than-average risk for having
a baby with a certain defect, further tests may be used for diagnosis.
Most women with abnormal screening tests have normal babies.
Your health care provider may offer you a maternal serum screening
test. The decision to have one of these tests is a personal one.
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Who
Should Be Tested
Maternal serum
screening tests can find a higher-than-average risk of open neural
tube defects (ONTDs) and Down syndrome. Diagnostic tests can
usually find the problem itself and will be offered to women who
have abnormal screening test results. If a woman is already at an
increased risk of having a baby with one of these problems, she
may be offered the diagnostic test first rather than having the
screening test. These risk factors may include:
. Being 35
years old or older when the baby is due
. Family or personal history of birth defects
. Previous child with a birth defect
. Use of certain medicines around the time of conception
. Insulin-dependent diabetes prior to pregnancy
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Maternal
Serum Screening Tests
Alpha-Fetoprotein
Test
Alpha-fetoprotein
(AFP) is a protein produced by a growing fetus. It
is present in amniotic fluid, fetal blood, and, in smaller amounts,
in the woman's blood.
The AFP test is usually performed at 15-20 weeks of pregnancy.
This is when the test is most accurate. For the test, a small
amount of blood is taken from a vein in the woman's arm. Results
are usually available in about a week.
Triple Screen Test
Adding certain tests to the AFP test can give more information
about your risk of having a baby with Down syndrome than the
AFP test alone. This is called a triple screen test.
Besides measuring AFP, a triple screen test measures other substances
in the woman's blood that come from the pregnancy. Two that
might be measured are human chorionic gonadotropin (hCG) and
estriol. hCG is a hormone produced by the placenta. Levels of
hCG are higher than normal in most pregnancies with a fetus
with Down syndrome. Estriol is produced mostly in the
placenta and in
the liver of the fetus. Estriol levels are lower than normal
in most pregnancies with a fetus with Down syndrome.
A triple
screen test is also performed at 15-20 weeks of pregnancy. As
in the AFP test, a small amount of blood is taken from a vein
in the woman's arm. Usually the same blood sample is used for
all the tests. Results are usually available within a week.
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What
Do the Test Results Mean?
Maternal serum
screening tests, such as the alpha-fetoprotein test and the multiple
marker screening test, assess your risk of having a baby with a
birth defect. Amniocentesis
is a diagnostic test--that is, it shows with a high degree of accuracy
whether your fetus has certain birth defects. This is possible because
the amniotic fluid contains cells from the fetus that have the same
genetic makeup as the fetus. A small amount of this fluid is taken
from the sac surrounding the fetus and tested. You may be offered
amniocentesis if your chances are higher than normal for having
a baby with a birth defect that the test can detect well.
Finally
. . .
Maternal
serum screening tests can give you an idea of your own risk for
having a baby with birth defects. A normal test result does not
guarantee that your child will not have birth defects. It
may reassure you that the risk is not increased. An abnormal test
result does not necessarily mean that your child will have birth
defects, but further testing should be considered.
It is your
decision whether to have the test. Some women find having the
test to be reassuring. Others would rather not have the information.
The results of these tests can help some women make decisions
about their options.
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