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Alpha-Fetoprotein Test
This presently is an optional blood test
performed between the 16th and 18th weeks of pregnancy. This special
blood test detects neural tube defects. Neural tube defects are
abnormalities in the brain and spinal cord of the fetus.
Defects in the central nervous system
occur when the neural tube (the brain and spinal cord tissues) fail to
close as the fetus develops. When the brain and spinal cord are exposed
directly to the amniotic fluid which surrounds the baby,
it
is called an “open defect.” Sometimes the
poorly developed neural tube is covered by skin or bone, referred to as
a “closed defect.”
The two
common neural tube defects are anencephaly and spina bifida. Babies with
anencephaly are born with deformities of the head and brain and die
soon after birth. Those born with spina bifida can live a long time, but
may suffer paralysis in the lower body and legs. Also, there is often a
lack of bladder and/or bowel control which maybe treated with surgery.
The chance of producing a child with a
neural tube defect is 1-2 for every 1,000 live births. Babies born to
mothers who have had a previous child with the same problem are at
greatest risk, as are those with parents or grandparents who have neural
tube defects.
As with all prenatal tests, a normal test
does not guarantee a normal baby at birth. About 20 percent of the
infants born with neural tube defects have normal alpha-fetoprotein (AFP)
levels. Most of these are closed defects which are typically less
severe.
Conversely, an initial abnormal test
reading does not mean the fetus has a neural tube defect. Abnormal
levels of AFP are frequent, occurring in about 50 of every 1,000 women
tested. Only one or two of those 50 actually have a neural tube problem.
A high AFP may be due to a miscalculation of the baby’s age, or due to
twins in the womb. Various other temporary fetal conditions can cause an
elevated AFP reading. Second AFP tests are normal in about half of those
who are retested.
If a second test also indicates an
abnormal AFP, a sonogram is usually given to determine the fetal age,
look for more than one fetus, or scan for neural tube defects and other
abnormal conditions which may be responsible for the elevated test.
If the sonogram shows a single fetus at
the approximate age determined by the initial due date with no fetal
abnormalities, an amniocentesis is performed. An abnormally high level
of AFP in the amniotic fluid indicates a 90 percent chance that a
serious problem is present.
An abnormally low AFP reading may indicate
that there is a chromosomal problem, such as Downs Syndrome (Mongolism).
An amniocentesis can resolve this issue.
TriScreen
Test
This optional test combines the
alpha-fetoprotein results with two other blood substances to give us the
triscreen test. The results are obtained from a blood sample from the
mother at 16-18 weeks of pregnancy. The majority of neural tube defects
and two of three babies with Down’s Syndrome can be detected with this
test. Further testing such as sonography and/or amniocentesis may be
required with an abnormal test.
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