Prenatal genetic testing gives parents-to-be information about whether their fetus has certain genetic disorders.
Genetic disorders are caused by changes in a person’s genes or chromosomes. Aneuploidy is a condition in which there are missing or extra chromosomes. In a trisomy, there is an extra chromosome. In monosomy, a chromosome is missing. Inherited disorders are caused by changes in genes called mutations. Inherited disorders include sickle cell disease, cystic fibrosis, Tay-Sachs disease, and many others. In most cases, both parents must carry the same gene to have an affected child.
There are two types of prenatal tests for genetic disorders:
- Prenatal screening tests: These tests can tell you the chances that your fetus has an aneuploidy and a few other disorders. This FAQ focuses on these tests.
- Prenatal diagnostic tests: These tests can tell you whether your fetus actually has certain disorders. These tests are done on cells from the fetus or placenta obtained through amniocentesis or chorionic villis sampling (CVS).
Screening tests can tell you your risk of having a baby with certain disorders. They include carrier screening and prenatal genetic screening tests:
- Carrier screening is done on parents (or those just thinking about becoming parents) using a blood sample or tissue sample swabbed from inside the cheek. These tests are used to find out whether a person carries a gene for certain inherited disorders. Carrier screening can be done before or during pregnancy.
- Prenatal genetic screening tests of the pregnant woman’s blood and findings from ultrasound exams can screen the fetus for aneuploidy; defects of the brain and spine called neural tube defects (NTDs); and some defects of the abdomen, heart, and facial features.
Results of blood screening tests for aneuploidy are reported as the level of risk that the disorder might be present:
- A positive screening test result for aneuploidy means that your fetus is at higher risk of having the disorder compared with the general population. It does not mean that your fetus definitely has the disorder.
- A negative result means that your fetus is at lower risk of having the disorder compared with the general population. It does not rule out the possibility that your fetus has the disorder.