Down Syndrome

Down syndrome can be diagnosed at birth, when the characteristic physical signs of Down syndrome are noted and chromosome analysis can also be performed to confirm the diagnosis and determine the recurrence risks.

At-risk pregnancies are referred for genetic counseling and prenatal diagnosis. Screening tests are available during a pregnancy to determine if the fetus has Down syndrome. During 14–17 weeks of pregnancy, a substance called AFP (alpha-fetoprotein) can be measured. AFP is normally found circulating in a pregnant woman's blood, but may be unusually high or low with certain disorders. Carrying a baby with Down syndrome often causes AFP to be lower than normal. This information alone, or along with measurements of two other hormones, is considered along with the mother's age to calculate the risk of the baby being born with Down syndrome.

A common method to directly determine whether the fetus has Down syndrome, is to test tissue from the fetus. This is usually done either by amniocentesis, or chorionic villus sampling (CVS). In amniocentesis, a small amount of the fluid in which the baby is floating is withdrawn with a long, thin needle. In chorionic villus sampling, a tiny tube is inserted into the opening of the uterus to retrieve a small sample of the chorionic villus (tissue that surrounds the growing fetus). Chromosome analysis follow both amniocentesis and CVS to determine whether the fetus is affected.

Once a couple has had one baby with Down syndrome, they are often concerned about the likelihood of future offspring also being born with the disorder. In most cases, it is unlikely that the risk is greater than other woman at a similar age. However, when the baby with Down syndrome has the type that results from a translocation, it is possible that one of the two parents is a carrier of that defect. When one parent is a carrier of a particular type of translocation, the chance of future offspring having Down syndrome is increased. The specific risks can be estimated by a genetic counselor.