Dr. MJ Bazos MD,
Amniocentesis and CVS
What is prenatal
Prenatal diagnosis is a way for your doctor to
tell if your fetus has a problem such as Down syndrome. Amniocentesis and
chorionic villus sampling (CVS) are tests that help find genetic disorders
before birth. Some parents are at increased risk of having a baby with a genetic
disorder or other problem and may want to have one of these tests. Knowing about
problems before the baby is born may help parents make decisions about health
care for their infant. Certain problems can be treated before the baby is born,
while other problems may need special treatment right after delivery. In some
cases, parents may decide not to continue the pregnancy.
Do all pregnant
women have these tests?
No. Amniocentesis or CVS may be considered in
situations in which the parents are at increased risk of having an infant with a
genetic disorder. The tests may be useful if you are 35 years of age or older by
the time your baby is due. Women over age 35 have an increased risk of having a
baby with a chromosome abnormality, such as Down syndrome. The tests may also be
useful if you have previously had a child with Down syndrome or another
disorder, such as spina bifida. The test may also be helpful if you or your
partner are known carriers of a genetic disorder, such as cystic
During amniocentesis, a sample of amniotic fluid
(the fluid around the baby) is removed from your uterus and sent to a laboratory
for evaluation. Amniocentesis is performed by inserting a thin needle through
your abdomen into your uterus (womb) and withdrawing a small amount of fluid.
Your body will make more fluid to replace the fluid that is taken out. The baby
will not be hurt during the procedure. Some women feel mild cramping during or
after the procedure. Your doctor may tell you to rest on the day of the test,
but usually you can resume normal activity the next day.
How is CVS
CVS is performed by removing a small sample of
the placenta (nourishment for the baby) from the uterus. It is removed with
either a catheter (a thin tube) or a needle. Local anesthesia is used for this
test. The sample of placenta may be obtained through the cervix. A catheter is
inserted into the vagina and through the cervix and the sample is withdrawn. The
sample also can be obtained by inserting a needle into the abdomen and
withdrawing some of the placenta. Most women feel fine after the test, although
some may have mild bleeding (spotting) afterward. CVS is usually done between
the 10th and 12th weeks of pregnancy.
When are the
Amniocentesis is usually performed during the
15th week of pregnancy or later. CVS is usually performed between the 10th and
12th weeks of pregnancy.
Is one test
better than the other?
There are some situations in which amniocentesis
is more appropriate than CVS. Amniocentesis is preferred if you have previously
had a baby with a neural tube defect or if you or your partner have a neural
tube defect. (CVS doesn't detect neural tube defects.)
Amniocentesis may be better if you have abnormal
results of blood tests (such as the alpha-fetoprotein test). The blood screening
test may show that your infant is at greater risk of having a neural tube defect
or a disorder such as Down syndrome. CVS may be better if you and your doctor
want to know the test results during your first trimester.
Are there risks
involved with these tests?
Amniocentesis and CVS carry a small risk of
miscarriage. Results of some studies suggest that in a small number of cases,
CVS may cause defects in the infant's fingers or toes. However, this only
appears to occur if the test is done before the 9th week of pregnancy. Your
doctor will talk to you about the risks, benefits and if the benefits of knowing
about a potential disorder are greater than the risks.