Amniocentesis is a diagnostic test offered to women over 35 during pregnancy.
An amniocentesis (amnio) is the most commonly used test used to diagnose Trisomy 18 and other chromosomal problems. (See How Trisomy 18 is diagnosed for an explanation of the difference between screening and diagnostic tests.) It is typically offered to all pregnant women over age 35, to those with previous pregnancies having chromosomal disorders, those with family history of a genetic disorder, or those having abnormal AFP or Level II ultrasounds. The amnio is usually performed at 15-18 weeks of pregnancy, although it can also be performed later.
There is a small risk of miscarriage, typically 1 in 200. If an AFP or quad screen result shows a risk of Trisomy 18 of greater than 1:100, level 2 ultrasound results are usually used to determine if the doctor will recommend an amnio. If an obvious problem is found on ultrasound, an amnio is typically recommended, and if only a single soft marker is found, the possibility of amnio is usually discussed with the parents since the risk of having Trisomy 18 must be weighed against the risk of the amnio itself.
The amnio results typically take 10 days or more, but there is a FISH test that can give preliminary results in a few days. For more information about the FISH test, see the Aneuvysion (the patented name for the FISH test) Patient Guide. You will need the free Adobe Acrobat reader to view this file. From the amnio, they do a karyotype of the baby's cells, which will clearly show the extra eighteenth chromosome if it is present. For a good description of all of these and other prenatal testing options see Prenatal Genetic Testing from the Genetics and Public Policy Center.
What if my amnio shows that my child has Trisomy 18?
If you have a positive diagnosis of Trisomy 18, you will be faced with many decisions. We have created a section to help you face those decisions. See Making Decisions after a Prenatal Diagnosis for more help.