If any one of these factors is reported inaccurately, a false positive or false negative result may occur. Down syndrome is caused by a change in the baby's chromosomes. These are the rod-shaped structures in the cells of the body that contain all the genes, or instructions for development. Most babies with Down syndrome have an extra copy of the 21st chromosome. Individuals with Down syndrome usually have mild to moderate mental retardation and a typical facial appearance.
Heart defects, abnormalities in the digestive system, and frequent infections are also common.
The risk for Down syndrome is increased when AFP and uE3 levels are low and the hCG level is high. Neural tube defects are a class of birth defects that occur in the spine and head. Spina bifida describes an opening in the spine, which can cause nerve damage. Anencephaly refers to an opening in the back of the head and incomplete formation of the brain, which is not compatible with life after birth. The risk for neural tube defects is increased when the AFP level is elevated.