What are karyotype tests?
Karyotype tests look closely
at the chromosomes inside your cells to see if there is anything out of the
ordinary. They are often done during pregnancy to check for problems with the
baby. This type of procedure is also called genetic or chromosome testing or
cytogenetic analysis.
When you are pregnant, your
doctor may do several prenatal screening tests to check for certain genetic and
chromosomal disorders. You will probably be offered one in the first trimester
and another in the second trimester.
Most women get results that
are within the normal range and do not require further testing. But if any of
your screening tests show signs of problems, your doctor will suggest follow-up
testing so you can know for sure if your growing baby has genetic or
chromosomal problems.
Uses for karyotype testing
Humans have 46 chromosomes.
Babies inherit 23 from their mother and 23 from their father.
Sometimes babies have an extra
chromosome, a missing chromosome, or an abnormal chromosome. Karyotype tests
will show if any of these events happened to your child. The most common things
doctors look for with karyotype tests include:
- Down syndrome (trisomy 21). The child has an extra third chromosome 21. This affects
his appearance and his learning.
- Edwards syndrome (trisomy 18). The child has an extra 18th chromosome. These babies
usually have many problems and most of them do not live more than a year.
- Patau syndrome (trisomy 13). The child has an extra 13th chromosome. These children
usually have heart problems and severe mental impairment. Most will not live
more than a year.
- Klinefelter's syndrome. The boy has an extra X chromosome (XXY). They may go
through puberty more slowly and not have children.
- Turner syndrome. The girl has a missing or damaged X chromosome. It causes
heart problems, neck problems, and short stature.
Karyotype tests can be used
for more than just detecting birth defects.
If you have had difficulty
getting pregnant or have had multiple miscarriages, your doctor may want to
check to see if you or your partner have a chromosome problem.
- You can find out if you have a disorder that you could pass on to your child.
- They can test a stillborn baby to see if there's a genetic problem.
- They can find the cause of certain physical or developmental problems in your child or young child.
- Chromosomal testing can show if a newborn is a boy or a girl in rare cases where it is not clear.
- Some cancers can cause chromosomal changes. Karyotype tests will help you choose the right treatment.
Types of karyotype tests
Karyotype tests can only be
done during certain weeks of pregnancy. Your doctor will suggest a karyotype
test that is right for you based on the stage of your pregnancy and your risks.
Your baby is more likely to
have chromosomal problems if:
- You’re 35 or older.
- You have another child or a family member who has a chromosome disorder.
- You or your partner have something unusual in your chromosomes.
- You have a history of miscarriages or had a baby that was stillborn.
Tests include:
Chorionic villus sampling. Doctors use a long needle to take a small sample of your
child's cells from the chorionic villi, which are tissues in the chorion.
Doctors send these cells to a
lab for further testing. The results can show if your child has Down syndrome,
trisomy 13, trisomy 18, or other genetic problems.
If your doctor says you need
to take a chorionic villus sampling, you can do so at 10-13 weeks. There is a
possibility that this could lead to a miscarriage. This happens to 1 in 100
women who take the test. There is also a risk to the baby, so doctors only
recommend it if there's a good chance your baby has a problem.
Amniocentesis. Doctors
get samples of your baby's cells by taking a small amount of amniotic fluid
(the fluid that surrounds your baby in the womb) with a long needle that they
stick into your abdomen. They send the cells to a lab for further testing.
The results of the test can
show if your child has any genetic problems that can be detected by analysis of
chorionic villus samples, as well as certain neural tube defects, which are
serious problems that can affect your child's brain or spine.
If your doctor says you need
an amniocentesis, you can do it between 15 and 20 weeks. There is a chance of
miscarriage, but it is less than with a chorionic villus biopsy. This only
happens to 1 in 200 women who have the procedure.
Bone marrow aspiration and biopsy. If you are having a chromosome test because of cancer or
a blood disorder, your doctor may need to take a sample of your bone marrow.
They usually take it from the thigh bone with a special needle when you are
numb but awake.
Risks of karyotype testing
The procedures used to collect
cells for karyotype testing carry certain risks. Chorionic villus sampling or
amniocentesis can cause miscarriage. You may also have heavy bleeding or
infection from aspiration and bone marrow biopsy.
Karyotype test results
When the lab returns your
results, they have examined your child's chromosomes. So the results are final:
either your child has a genetic problem or it doesn't.
This is different from earlier screening tests, which could only tell if the risk of a problem was high or low. Your doctor will discuss the test results with you.