Screening vs. diagnostic testing—what’s the difference?

If you are pregnant, you know that every visit to your prenatal care provider involves a number of tests. At each prenatal checkup, your provider checks your weight, blood and urine. But in addition to these routine tests, you will also be offered prenatal tests that can assess your risk to have a baby with certain birth defects.

The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women, regardless of age, be offered prenatal testing for Down syndrome and some other birth defects. There are two different types of tests that you can have: a screening test or a diagnostic test. It’s important to understand the difference between a screening test and a diagnostic test. Screening tests help evaluate the risk for certain birth defects, but they cannot diagnose a birth defect. Screening tests pose no risk to mother or baby. Diagnostic tests are highly accurate at diagnosing or ruling out specific birth defects. However, these tests may pose a very small risk of miscarriage.

Screening tests offered during pregnancy include:

• Cell-free fetal DNA testing (also called noninvasive prenatal screening or testing): Some of your baby’s cells can be found in your blood. This test uses a sample of your blood to look at your baby’s DNA to check for certain genetic conditions. You can have this test after 10 weeks of pregnancy. This test is not recommended for women who aren’t likely to have a baby with a birth defect or who are pregnant with multiples.
• First-trimester screening: Using a blood sample and ultrasound measurements, first-trimester screening can tell you if your baby is at risk for some birth defects, like Down syndrome. It is usually done between 11 to 13 weeks of pregnancy.
• Maternal blood screening: This test measures four substances in your blood to determine the chance that a woman has a baby with certain birth defects like Down syndrome and neural tube defects. The test is done between 15 to 20 weeks of pregnancy.

Diagnostic tests are done by obtaining samples of your baby’s actual cells and therefore are more invasive. Diagnostic testing can detect most birth defects caused by a change in the number or shape of chromosomes. And testing for many inherited disorders can be done as well. However, not all birth defects can be detected.

Some women may choose to have diagnostic testing done instead of screening tests based on their age or family history. Other women may choose to start with a screening test and then, depending on the results, decide whether to have a diagnostic test. Prenatal diagnostic tests do carry a small risk of miscarriage (about 1 in 300-500).

• Chorionic villus sampling (also called CVS): During this procedure, the doctor removes a small sample of tissue from the placenta. This can either be done transvaginally or transabdominally. You can get CVS at 10 to 12 weeks of pregnancy.
• Amniocentesis (also called amnio): Your health care provider will use a needle to remove a small amount of amniotic fluid. Amnio is done between 15 to20 weeks of pregnancy.

Remember, all of these tests are optional. Make sure that you discuss your questions and concerns with your health care provider.

Questions? Send them to AskUs@marchofdimes.org.

 

Tags: amniocentesis, birth defects, chorionic villus sampling, CVS, Down Syndrome, maternal blood screening, prenatal tests, ultrasound