How does the Down syndrome blood test work?
The Down syndrome blood test is a new prenatal screening test for women
at high risk of having a baby who has Down syndrome. The test is used
to evaluate your risk of carrying a baby who has Down syndrome. In some
cases, the Down syndrome blood test can help you avoid testing that's
During the Down syndrome blood test — which can
be done as early as week 10 of pregnancy — a maternal blood sample is
taken and sent to a lab. At the lab, fetal DNA circulating within the
blood is analyzed for extra chromosome 21 sequences. A higher than
expected number of chromosome 21 sequences indicates an increased risk
of trisomy 21, the most common cause of Down syndrome. Trisomy 21 occurs
when a person has three copies of chromosome 21, instead of the usual
two copies, in all of his or her cells. The Down syndrome blood test
doesn't specifically address other genetic or chromosomal conditions or
other rare causes of Down syndrome. Your health care provider will be
notified, however, if extra sequences of chromosome 13 (trisomy 13) or
chromosome 18 (trisomy 18, also called Edwards syndrome) are found.
Typically, test results are available within two weeks.
traditional prenatal screening test — such as first trimester screening
or the quad screen — indicates that you're at high risk of carrying a
baby who has Down syndrome, a more invasive test is typically offered to
confirm the diagnosis. For example:
Amniocentesis. During this
procedure, a sample of the fluid that surrounds and protects the baby
during pregnancy (amniotic fluid) is removed from the uterus for
Chorionic villus sampling (CVS). During CVS, a sample of
the wispy projections that make up most of the placenta (chorionic
villi) are removed from the placenta for testing.
CVS both carry a slight risk of miscarriage. You might be able to avoid
amniocentesis or CVS, however, by taking the Down syndrome blood test.
If the Down syndrome blood test results are normal, you might not need
further testing. If the Down syndrome blood test detects a high risk of
trisomy 21, amniocentesis or CVS will be offered to confirm the results.
If you have risk factors for having a baby who has Down syndrome —
you're an older mother or you have a family history of Down syndrome —
ask your health care provider about the Down syndrome blood test. Also
ask about the blood test if you've gotten positive or worrisome results
from another prenatal screening test. Your health care provider or a
genetic counselor can help answer any questions you might have and
discuss what the results might mean for you and your family.