Trisomy 18 Testing

Trisomy 18 occurs when an error in cell division results in three copies (instead of the normal two copies) of chromosome 18. Trisomy 18 is also known as Edwards syndrome. Fewer than 10% of affected babies survive to their first birthday, and those who do survive are affected by severe developmental delays. Prenatal diagnosis is available for trisomy 18.

Screening blood tests, such as the quad screen or triple screen, are administered to pregnant women between the 15th and 20th weeks of pregnancy. These screening tests do not diagnose trisomy 18 or other conditions but instead identify women at higher risk of having an affected baby. Women who are identified as having a high risk on a screening test can undergo additional and more specific testing. A level 2 ultrasound is another type of screening test that can be done if a woman is found to be at higher risk. This ultrasound examination looks for abnormalities in the fetus that might suggest trisomy 18, but it does not definitively diagnose the condition.

To firmly establish the diagnosis, chromosomal tests are required. A sample of amniotic fluid can be withdrawn by amniocentesis and analyzed in the laboratory for the presence of the extra copy of chromosome 18. Chorionic villus sampling (CVS) is another method to retrieve a tissue sample for chromosome analysis, but this must be performed between 10 and 12 weeks of gestation.