There are two types of prenatal genetic tests: screening and diagnostic.
Screening tests show the chance that a developing baby (fetus) has a certain genetic condition. It can't tell for sure that your baby has a problem. If the test result is "positive," it means that your baby is more likely to have that condition. So your doctor may want you to have a diagnostic test to make sure. If the screening test result is "negative," it means that your baby probably doesn't have that genetic condition. But it doesn't guarantee that you will have a normal pregnancy or baby.
Diagnostic tests show if the developing baby has a certain genetic condition.
Examples of tests used for prenatal screening include:
If prenatal screening test results show an increased risk of problems, further diagnostic genetic testing (karyotype) can be used to examine the size, shape, and number of chromosomes. A karyotype can be done on cells taken from the placenta (chorionic villus sampling) in the late first trimester or from the amniotic fluid (amniocentesis) in the second trimester. Extra, missing, or abnormal positions of chromosome pieces can cause problems with growth, development, and body functions.