What is Down Syndrome?

About Down Syndrome

In every cell in the human body there is a nucleus, where genetic material is stored in genes. Genes carry the codes responsible for all of our inherited traits and are grouped along rod-like structures called chromosomes. Typically, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21.

This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome. A few of the common physical traits of Down syndrome are low muscle tone, small stature, an upward slant to the eyes, and a single deep crease across the center of the palm – although each person with Down syndrome is a unique individual and may possess these characteristics to different degrees, or not at all.

Prenatal Diagnosis

Two types of procedures are available to pregnant women: screening tests and diagnostic tests. The screening tests estimate the risk of the baby having Down syndrome. Diagnostic tests tell whether or not the baby actually has Down syndrome.

Screening Tests

• Maternal serum tests, such as the triple screen or quad screen, measure quantities of various substances in the blood. These tests are usually done between 15 and 20 weeks gestation.
• Newer non-invasive tests have been developed which look for genetic material from the baby in the mother’s blood.  They can be done earlier in the pregnancy and are more accurate than other screening tests, but they are currently recommended only for “high risk” pregnancies.
• Sonograms (ultrasounds) are usually performed in conjunction with other screenings. These can show some physical traits that are helpful in calculating the risk of Down syndrome.
• Screening tests do not accurately confirm the diagnosis of Down syndrome. In fact, false positives and false negatives frequently occur.

Diagnostic Tests

Three diagnostic tests are currently available:

• Chorionic Villus Sampling (CVS) is conducted between 10 and 12 weeks.  In this test, a small piece of the placenta (chorionic villus) will be taken and analyzed.
• Amniocentesis is performed between 14 and 20 weeks.  A small sample of amniotic fluid is drawn, and cells in the fluid are tested.
• Percutaneous Umbilical Blood Sampling (PUBS) is performed after 18 weeks.  This test examines blood from the umbilical cord.

For more information on Down Syndrome please visit The National Down Syndrome Society

For facts and information on Down Syndrome with Spanish translation available please visit National Association for Down Syndrome

To read some of the latest research about Down syndrome please visit STAT News