A Down syndrome test is used to discover whether an unborn baby has Down syndrome, a genetic condition caused by an extra copy of chromosome 21. Down syndrome occurs in about 1 out of every 1,000 live births.
Babies with Down syndrome have a range of physical and intellectual differences. They’re also at higher risk for certain health conditions, such as cancer and heart defects.
Prenatal testing for Down syndrome is entirely optional. The decision to undergo prenatal testing for Down syndrome can be a deeply personal one.
Screening Tests During Pregnancy
Screening tests measure the risk that your baby will have Down syndrome. They don’t result in a diagnosis — they tell you if a condition, like Down syndrome, might be present.
Prenatal screening tests for Down syndrome include:
- Blood Test This test measures levels of certain hormones and proteins in your blood that may indicate a genetic condition such as Down syndrome. Doctors perform this test during the first or second trimester, along with standard prenatal testing.
- Ultrasound A nuchal translucency test uses ultrasound to measure fluid on your baby’s neck. More fluid than usual may indicate Down syndrome. Doctors typically perform this test during the first trimester.
Diagnostic Down Syndrome Tests
If your screening tests show a high likelihood of having a baby with Down syndrome, or if you have other risk factors, your doctor may recommend diagnostic testing. The American Academy of Obstetrics and Gynecology recommends that providers offer all pregnant women, regardless of age and risk factors, the option of diagnostic testing first, bypassing screening tests.
Down syndrome diagnostic tests include:
- Amniocentesis Your provider examines your amniotic fluid for irregularities, specifically three copies of chromosome 21. Amniotic fluid is the liquid that surrounds and protects the fetus in the womb. It contains some cells shed by the fetus so doctors can examine the chromosomal makeup of these cells in order to determine the makeup of the fetus as well. Doctors perform this screening test during the second trimester.
- Chorionic Villus Sampling (CVS) Your provider takes cells from your placenta to examine your baby’s chromosomes. The placenta is an organ that forms in the womb during pregnancy and contains the same genetic material as the fetus. This test is done during the first trimester.
- Percutaneous Umbilical Blood Sampling (PUBS), Also Called Cordocentesis Doctors test fetal blood from your baby’s umbilical cord for genetic information. This is the most accurate prenatal test for Down syndrome, and is sometimes used to confirm amniocentesis or CVS results. The test usually occurs between 18 and 22 weeks of pregnancy.
