Since Down syndrome is such a unique group of characteristics, physicians can sometimes determine simply by physical examination whether a baby has Down syndrome. To confirm the physical findings, a small blood sample can be taken and the chromosomes can be analyzed to determine the presence of an extra 21 chromosome. This information is important in determining the risk in future pregnancies.
Chromosomal abnormalities such as Down syndrome can often be diagnosed before birth by analyzing cells in the amniotic fluid or from the placenta. Fetal ultrasound during pregnancy can also give information about the possibility of Down syndrome, but ultrasound is not 100 percent accurate, since many babies with Down syndrome may look the same on ultrasound as those without Down syndrome. A chromosome analysis, whether performed on a blood sample, cells from the amniotic fluid or placenta, is over 99.9 percent accurate.