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What
is genetic screening?
Genetic screening
is a process used to find out what diseases or birth defects a child
might inherit from his or her parents. It is best to do genetic
screening before you get pregnant. It may also be done at
your first prenatal visit and later in your pregnancy. Chorionic
villus sampling (CVS) and amniocentesis are examples of genetic
screening tests done during pregnancy.
A couple
planning to have a baby may be concerned about illnesses in their
families that a child might inherit. They can request genetic
counseling from their health care provider. The health care
provider or a genetic counselor will ask for a detailed family
history of diseases, disorders, and birth defects. Some
blood tests might also be done. If a woman is already pregnant,
tests to examine the baby's condition and chromosomes might be
performed.
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When
is it used?
Some of the
most common reasons for couples to have genetic screening include:
- The mother
will be 35 years old or older at the time of delivery.
- A couple
has already had a child with an inherited disease or birth defect.
- The couple
has had stillbirths (babies born deceased after 28 weeks of
pregnancy) or several miscarriages.
- An abnormality
has already been found, such as an abnormal level of serum alpha
fetoprotein (AFP), or too much or too little amniotic fluid
around the baby. AFP is a protein made by the fetus.
AFP measurements in amniotic fluid are used for early diagnosis
of certain nervous system or neural tube defects, such as spina
bifida.
- They
have a family history of thalassemia, a disorder in which a
person's red blood cells are abnormal. Thalassemia is most common
in people from southeast Asia, China, and Mediterranean countries
(for example, Italians and Greeks).
- They
have a family history of Tay-Sachs disease, a brain disorder
that can cause early death. This disease is most common
in Jews who have an Eastern European Ashkenazic ancestry.
- They
have a family history of other inherited disorders, such as
hemophilia (a blood clotting disorder) or cystic fibrosis.
- They
have a family history of sickle cell anemia, a disorder in which
a person's red blood cells are abnormal. This disorder
is most common in North American blacks.
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How
do I prepare for genetic screening?
Find out the
medical history of family members, including details of any inherited
diseases in either the mother's or father's families. Specifically
ask your parents and your partner's parents if there have been any
children with mental retardation, disabilities, or other abnormalities
in their families.
Also be prepared to report the following personal information:
- any past
miscarriages
- any exposure
to chemicals, radiation (including x-rays), or other environmental
hazards (such as through your work or hobbies) before or during
pregnancy
- any history
of drug or alcohol use
- any prescription
or over-the-counter drugs taken by you before you became pregnant
or before you knew you were pregnant.
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What
happens during the procedure?
The health care
provider or genetic counselor will review your partner's and your
family and personal medical histories. In addition the following
procedures are used to look for birth defects and inherited diseases:
- maternal
blood tests at 15 to 19 weeks in the pregnancy to check the
level of alpha fetoprotein (If it is high or low, the
health care provider may perform other tests to look for defects
in the nervous system called neural tube defects, such as spina
bifida or chromosome defects).
- ultrasound
scans after the 15th week of pregnancy to check the baby for
birth defects of the brain, heart, spine, legs, arms, or other
organs
- chorionic
villus sampling (CVS) between the 9th and 12th weeks pregnancy
to test a sample of tissue from the placenta for chromosomal
abnormalities
- amniocentesis
at the 16th week of pregnancy to test the amniotic fluid for
chromosomal abnormalities and other substances such as alpha
fetoprotein.
What
happens after the screening?
The health care
provider will discuss the results with you. If there is a
problem, your health care provider or genetic counselor should discuss
the treatment options. The treatment you select is a very
personal choice. Your options should be clearly defined and
discussed at length.
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