Therefore, it does not replace genetic testing. If the test is abnormal, it means that you are at increased risk of having a child with these problems. It is not designed to make a diagnosis. To do this, an ultrasound and amniocentesis are recommended. Amniocentesis is usually performed at 16-18 weeks.
Amniotic fluid is drawn from around the fetus under ultrasound guidance.
The cells floating in the fluid are tested for their genetic make up and the fluid itself can be tested for alpha-fetoprotein, a protein which can detect open spine. Ultrasound is performed at the same time to avoid injury to the fetus and to look for any developmental anomalies, which cannot be detected by chromosomal evaluation. Chorionic Villus Sampling is a genetic test that can be performed before 12 weeks. Cells from the edge of the placenta are removed. Since they have the same genetics as the fetus, the genetic make up of the fetus can be determined through these cells.