health

Most babies with Down syndrome are diagnosed at birth on the basis of
their physical features. The diagnosis can be confirmed by a blood test
and karyotype (an analysis of a person’s chromosomes).
The diagnosis can also be made before birth. There are two types of
prenatal tests: screeners and diagnostic tests. Screeners only estimate the
baby’s risk of having Down syndrome. Given between the fifteenth and
twentieth weeks of pregnancy, screening tests include a blood test and an
ultrasound imaging test. These tests are only about 60 percent accurate,
however. Diagnostic tests that are about 98 percent accurate include
chorionic villus sampling (CVS), done between the ninth and fourteenth
weeks of pregnancy; amniocentesis, which can be done at the same time
that the screeners can be given; and testing blood samples taken from the
baby’s umbilical cord, done after the eighteenth week of pregnancy.