Everybody is allowed to have the test but women choose to do the test when have an increased chance of passing on a genetic disorder.
Some example are Down syndrome, Patau's syndrome and Edwards' syndrome. The test can also identify hundreds of genetic disorders, including sickle-cell disease and cystic fibrosis.
The risk is determined by many factors like the parents age, the family history, and if you have other children with a genetic disorder.
Everybody is offered the chance to have the test done.