Down Syndrome Screening
Special Newsletter Article: "First
Trimester Down Syndrome Screening"
What is Down syndrome?
Down syndrome, also called “trisomy 21,”
occurs when there is an extra chromosome 21 present
in cells of the fetus. All of our genes are packaged
into chromosomes which control all of our genetic
characteristics. Chromosomes are present in pairs.
Normally, a baby is born with 46 chromosomes in
every cell, including two copies of the number
21 chromosome. A child with an extra chromosome
21 will have mental retardation and the characteristic
appearance of Down syndrome. About 40 percent
of children with Down syndrome are also born with
serious heart problems.
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Why should I consider being
screened for Down syndrome?
Down syndrome occurs in approximately 1 in 800
births. The older a woman is at the time of her
delivery, the greater the chance that she will
have a baby with Down syndrome. However, a woman
at any age can have a baby with Down syndrome.
By combining the results of the Multiple Marker
Screening (MMS) blood test, with your age and
other factors, you will receive a more accurate
assessment of your individual risk for having
a baby with Down syndrome.
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Should I have this test
if there has never been anyone in our family with
Down syndrome?
In over 95 percent of the families in which a
child is born with Down syndrome, there has never
been anyone else in the family with this problem.
A very small percentage of Down syndrome is inherited
in families. If you or your partner have a history
of Down syndrome in the family, please let your
doctor or genetic counselor know prior to having
this test.
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How accurately does
the MMS test screen for Down syndrome?
The MMS test detects approximately 70 percent
of Down syndrome pregnancies. If your test result
shows a risk for Down syndrome that is comparable
to the risk of a 35-year-old woman (a one in 270
chance), you will be counseled by your doctor
and our genetic counselors about the result. If
your test result shows a risk lower than that
of a 35-year-old woman, the report will be sent
to your doctor. If you wish to know the numerical
risk estimated by the test, please ask your doctor.
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What if the screening test
shows that my risk for Down syndrome is equal
to or greater than that of a 35-year-old woman?
The first step is to have an ultrasound examination.
Sometimes the blood test results are higher or
lower than we expect simply because your pregnancy
is not quite as far along or is more advanced
than we calculated by your last menstrual period.
In this case, the expected date of your delivery
and the interpretation of your test may be changed
after the ultrasound scan. If your risk for Down
syndrome is still increased, you may decide to
have an amniocentesis to test for Down syndrome
in the fetus. Amniocentesis is done by removing
a small amount of the amniotic fluid surrounding
the fetus and checking the chromosomes in the
cells that are in this fluid for the extra chromosome
21.
New Down syndrome screening tests continue to
become available and can be performed in the first
trimester. These tests include a combination of
enhanced ultrasound and blood tests. Talk to your
doctor about the latest technology available to
detect Down syndrome.
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If I am over 35 years of
age, should I consider having the MMS blood test
first to identify my risk of having a child with
Down syndrome before deciding about an amniocentesis?
The current recommendation for prenatal care of
women over 35 years of age is that they should
be offered amniocentesis. MMS is not a substitute
for amniocentesis. However, some patients find
that the MMS blood test helps them make a decision
regarding whether an amniocentesis is the right
choice for them. These options require more extensive
counseling by our genetic counselors before you
make this important decision. If you will be over
35 years of age when you deliver, please contact
one of our genetic counselors for more information.
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What if amniocentesis
shows that my baby will have Down syndrome or
some other serious birth defect?
If your baby is found to have a serious birth
defect, you will receive counseling about what
this will mean for you and your family.
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If my blood test is normal,
does that mean that my baby will be normal?
The MMS test identifies seven out of 10 cases
of Down syndrome (70 percent). Because MMS is
a screening test, some cases of Down syndrome
will be missed. The AFP part of the MMS test also
screens for neural tube defects, such as spina
bifida. There are other types of birth defects
that are not detected by this test. Therefore,
a normal test does not guarantee a normal baby.
Penn Ob/Gyn Care physicians and counselors provide
prenatal diagnosis, consultation and care at various
locations throughout the Philadelphia region.
Please call PennHealth at 1-800-789-PENN (7336)
for more information.
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