First Trimester Screening for Trisomy 21 (Down Syndrome) and Trisomy 18 -- Frequently Asked Questions

Get answers to common questions about perinatal care.

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.