There are two types of prenatal tests available to detect Down syndrome in a fetus: screening tests and diagnostic tests.
Screening tests estimate the risk that a fetus has DS
Diagnostic tests can tell whether the fetus actually has the condition.
Screening tests are noninvasive and generally painless. But because they can't give a definitive answer as to whether a baby has DS, mostly they're used to help parents decide whether to have more diagnostic tests.
Diagnostic tests are about 99% accurate in detecting Down syndrome and other chromosomal abnormalities. However, because they are performed inside the uterus, they are associated with a risk of miscarriage and other complications. For this reason, they are generally recommended only for women age 35 or older, those with a family history of genetic defects, or those who've had an abnormal result on a screening test. If you're unsure about which test, if any, is right for you, your doctor or a genetic counselor can help you sort through the pros and cons of each.