What Is Down Syndrome? What Causes Down Syndrome?

What is Down syndrome?

Down syndrome is a chromosomal disorder caused by an error in cell division that results in an extra 21st chromosome. The condition leads to impairments in both cognitive ability and physical growth that range from mild to moderate developmental disabilities. Through a series of screenings and tests, Down syndrome can be detected before and after a baby is born.

The only factor known to affect the probability of having a baby with Down syndrome is maternal age. That is, less than one in 1,000 pregnancies for mothers less than 30 years of age results in a baby with Down syndrome. For mothers who are 44 years of age, about 1 in 35 pregnancies results in a baby with Down syndrome. Because younger women generally have more children, about 75 - 80% of children with Down syndrome are born to younger women.

What causes Down syndrome?

Down syndrome occurs because of an abnormality characterized by an extra copy of genetic material on all or part of the 21st chromosome. Every cell in the body contains genes that are grouped along chromosomes in the cell's nucleus or center. There are normally 46 chromosomes in each cell, 23 inherited from your mother and 23 from your father. When some or all of a person's cells have an extra full or partial copy of chromosome 21, the result is Down syndrome.

The most common form of Down syndrome is known as Trisomy 21, a condition where individuals have 47 chromosomes in each cell instead of 46. This is caused by an error in cell division called nondisjunction, which leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception. Trisomy 21 accounts for 95% of Down syndrome cases, with 88% originating from nondisjunction of the mother's egg cell.

The remaining 5% of Down syndrome cases are due to conditions called mosaicism and translocation. Mosaic Down syndrome results when some cells in the body are normal while others have Trisomy 21. Robertsonian translocation occurs when part of chromosome 21 breaks off during cell division and attaches to another chromosome (usually chromosome 14). The presence of this extra part of chromosome 21 causes Down some syndrome characteristics. Although a person with a translocation may appear physically normal, he or she has a greater risk of producing a child with an extra 21st chromosome.

What are characteristics of people with Down syndrome?

Individuals with Down syndrome often have distinct physical characteristics, unique health issues, and variability in cognitive development. Physical characteristics include:

• Eyes that have an upward slant, oblique fissures, epicanthic skin folds on the inner corner, and white spots on the iris
• Low muscle tone
• Small stature and short neck
• Flat nasal bridge
• Single, deep creases across the center of the palm
• Protruding tongue
• Large space between large and second toe
• A single flexion furrow of the fifth finger

Individuals with Down syndrome usually have cognitive development profiles indicative of mild to moderate mental retardation. However, cognitive development in children with Down syndrome is quite variable. Children with Down syndrome often have a speech delay and require speech therapy to assist with expressive language. In addition, fine motor skills are delayed and tend to lag behind gross motor skills. Children with Down syndrome may not walk until age 4, but some will walk at age 2. Although many with the condition experience developmental delays, it is not uncommon for those with Down syndrome to attend school and become active, working members in the community.

Individuals with Down syndrome can have abnormalities affecting general health that may affect any organ system or bodily function. They have an increased risk for congenital heart defects, respiratory and hearing problems, Alzheimer's disease, childhood leukemia, epilepsy, and thyroid conditions. However, people with Down syndrome also have a lower risk of hardening of the arteries, diabetic retinopathy, and most kinds of cancer.

How is Down syndrome screened and diagnosed?

Some families have prenatal exams that indicate various possible problems, and others already know that they have an increased chance of having a child with Down syndrome. These families often receive screening and diagnostic tests for the condition. It is also standard for pregnant women older than 30 or 35 to receive genetic screens because the risk of having a child with Down syndrome is increased as women age.

Screening tests are used to estimate the risk that a fetus has Down syndrome, and diagnostic tests can tell whether the fetus actually has the condition. Screening tests are a cost-effective and less invasive way to determine if more invasive diagnostic tests are needed. However, unlike diagnostic tests, screening tests cannot give definite answers as to whether the baby has Down syndrome. Diagnostic tests, which are 99% accurate in detecting Down syndrome and other problems, are usually performed inside the uterus and carry an extra risk of miscarriage, fetal injury, or preterm labor.

Screening tests include:

• Nuchal translucency testing (at 11 to 14 weeks) - an ultrasound that measures clear space in folds of tissue behind the neck of a developing baby


• Triple screen or quadruple screen (at 15 to 18 weeks) - measures the quantities of normal substances in the mother's blood


• Integrated screen - combines first trimester screening tests (with or without nuchal translucency) and blood tests with second trimester quadruple screen


• Genetic ultrasound (at 18 to 20 weeks) - Detailed ultrasound combined with blood test results

Diagnostic tests include:

• Chorionic villus sampling (at 8 to 12 weeks) - analysis of a tiny sample of placenta obtained from a needle inserted into the cervix or the abdomen


• Amniocentesis (at 15 to 20 weeks) - analysis of a small amount of amniotic fluid obtained from a needle inserted into the abdomen


• Percutaneous umbilical blood sampling (after 20 weeks) - analysis of a small sample of blood from the umbilical cord obtained from a needle inserted into the abdomen

Down syndrome can also be diagnosed after a baby is born by inspecting the infant's physical characteristics as well as blood and tissue samples that are stained to show chromosomes grouped by size, number, and shape.