After age 45, the risk is roughly 1 in 25. Down syndrome is caused by a chromosome abnormality that is present when conception occurs. So there is no way to prevent Down syndrome in a person who has already been born. Some women may want to have their child screened for Down syndrome during early pregnancy. If screening detects Down syndrome in the child, some women may elect to have an elective abortion .
Genetic counseling is also useful for the families of affected children.
Down syndrome may be suspected at birth when certain physical features or defects are present. A chromosome analysis, however, confirms the diagnosis. There are also tests that screen for Down syndrome during pregnancy. A blood test known as the triple screen measures the levels of certain chemicals in a pregnant woman's blood. These chemicals are estriol, human chorionic gonadotropin, and alpha-fetoprotein .