First Trimester Screening for Trisomy 21 (Down Syndrome)
and Trisomy 18
What is first trimester screening?
First trimester screening is the combination of a maternal
blood test and an ultrasound measurement to screen for
chromosomal defects associated with significant mental
disability, Down syndrome, Trisomy 13, and Trisomy 18.
Down syndrome, Trisomy 13, and Trisomy 18 are chromosome
problems that happen randomly at the time of conception. These
conditions are relatively uncommon, but the risk increases with
a woman’s age.
The blood screen measures two pregnancy proteins in the
mother’s blood, free beta-HCG and PAPP-A. The ultrasound
measures the nuchal translucency, an area behind the baby’s
neck that contains fluid.
First trimester screening is performed between 11 weeks and
14 weeks gestation and is safe for the mother and baby.
How accurate is this screening?
First trimester screening can identify 80 to 90 percent of
pregnancies with Down syndrome and up to 97 percent of
pregnancies with Trisomy 18. The results are expressed in a
ratio, such as “the risk of Down syndrome is 1 in 200.” This
means that of 200 women with those same results, only one would
have an affected pregnancy.
It is important to realize that first trimester screening
cannot diagnose or rule out any specific condition.
What does an abnormal result mean?
An abnormal screen means that the estimated chance of having a
baby with Down syndrome or Trisomy 18 is higher than
expected.
Only a small number of women with an abnormal screen result
will have an affected pregnancy. If the screen does identify an
increased chance, genetic counseling and further testing will
be offered.
Tests that diagnose chromosome problems in pregnancy include
chorionic villi sampling (CVS) and amniocentesis. Each has a
small risk of complications that can lead to miscarriage.
Can this screen identify other
problems?
Although first trimester screening is specific to Down
syndrome, Trisomy 13, and Trisomy 18, other birth defects such
as heart defects can be associated with an increased nuchal
translucency.
For those who have an abnormal ultrasound measurement and
otherwise normal test results, targeted ultrasound in the
second trimester will be recommended.
How does this screen compare with screening in the
second trimester?
Maternal blood screening in the second trimester, offered
between 15 and 21 weeks gestation, measures multiple pregnancy
proteins to estimate the risk of Down syndrome, Trisomy 18, and
open neural tube defects (ONTDs).
ONTDs are openings along the fetal spine (open spina bifida)
and skull (anencephaly). About one to two babies per thousand
are affected with an ONTD.
Second trimester maternal blood screening can identify 60 to
80 percent of pregnancies with Down syndrome or Trisomy 18 and
80 percent of pregnancies with ONTDs.
First trimester screening cannot identify a risk of an ONTD.
If you elect first trimester screening, maternal serum AFP or
targeted ultrasound in the second trimester are
recommended.
Who should consider first trimester
screening?
First trimester screening is available to anyone who desires
early pregnancy screening for Down syndrome, Trisomy 13, and
Trisomy 18.
Or, a couple with an increased chance of these chromosome
problems (due to maternal age or family history) might find it
helpful to have more specific information about the pregnancy
before making decisions about diagnostic testing.
If you have decided to have amniocentesis or CVS, then this
test is not necessary.
Review the American College of Obstetricians and Gynecologists
position statement.
Back to First Trimester Screening
