Diagnostic tests for Down syndrome (trisomy 21) are considered invasive testing procedures.
Note: The American College of Obstetricians and Gynecologists (ACOG) recommends that pregnant women of all ages have the option of bypassing the screening test and choosing instead to have a diagnostic test for Down syndrome.
Diagnostic tests may be offered for several reasons, including:
- Will be age 35 or older at delivery
- Abnormal results obtained on screening tests or ultrasound
- Prior child or pregnancy with a birth defect
Diagnostic tests, which use a sample of fetal cells, include one or both of the following procedures:
- Chorionic villus sampling (CVS): CVS is usually done between 10 and 13 weeks of pregnancy. It is considered an invasive procedure since the procedure involves the collection of chorionic villus cell samples from the placenta, either via a needle insertion into the abdominal wall or a catheter in the vagina. Samples are analyzed for fetal chromosome abnormalities.
- Amniocentesis: Amniocentesis is performed between gestational weeks 15 and 20 and is also classified as an invasive procedure. A thin needle is placed into the abdominal wall and a sample of the amniotic fluid is collected. The sample is analyzed for fetal chromosome abnormalities.
Both CVS and amniocentesis are highly accurate for diagnosing Down syndrome and/or ruling out the condition. However, because they are invasive procedures, there are risks, including miscarriage, preterm labor, infection, or injury to the fetus or mother.
Both CVS and amniocentesis are discussed in more detail in the following sections.
