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.
