“Routine” amniocentesis is a prenatal diagnostic test performed at 15-20 weeks of pregnancy. Amniocentesis detects chromosome abnormalities and can also identify certain other genetic disorders in the baby. Additionally, amniocentesis can detect approximately 95 percent of neural tube defects (such as spina bifida and anencephaly).
The test involves placing a very fine needle through the woman’s abdomen (not through the navel) into the amniotic sac. A small amount of amniotic fluid is withdrawn from the sac and sent to the genetics laboratory for processing. Amniotic fluid contains fetal epithelial cells that carry the genetic material of the fetus.
Once the fluid is extracted, we separate the cells from the fluid and culture them. After growing the cells, we can look at the chromosomes and also analyze the DNA for suspected genetic disorders. In addition, we take the amniotic fluid and measure the concentration of alpha-fetoprotein which tests for neural tube defects (such as spina bifida) or ventral wall defects (such as gastroschisis).
It is important to note that the small amount of amniotic fluid that is taken for analysis is replenished very quickly by the baby. There are risks involved with the amniocentesis procedure. Therefore, a genetic counselor can discuss the benefits, risks, and limitations of prenatal diagnosis with individuals who may be indicated.