Genetic Testing Disease Risk Profiling Down Syndrome

Down syndrome (DS) is a condition caused by extra genetic material which results in physical and mental abnormalities. Also called Trisomy 21, it is a genetic condition that occurs when a baby is born with three copies of chromosome 21 instead of two.

Although it can be detected before birth, Down syndrome cannot be prevented. The severity of physical and medical problems of individuals with Down syndrome varies greatly. Some require constant care and others are more self-sufficient.

Children with Down syndrome usually have a flat facial profile, an upward slant of the eyes, small ears and a large or protruding tongue. They also usually have low muscle tone. They reach developmental milestones at a slower pace than other children. Growth is also slower, resulting in delayed speech and self-care skills. Most have mild to moderate intellectual impairment. About half the children have congenital heart defects.

Causes of Down Syndrome

No one understands why Down syndrome occurs. It affects about one in 800 babies. Physical and developmental delays are caused by having extra pieces of chromosome 21, either a complete extra copy or just part of the chromosome.

The chances of Down syndrome genes being passed on increase according to the age of the mother:

  • At age 30, the chance is one in 900.
  • At age 35, the chance is one in 350.
  • At age 40, the chance is one in 100.

Testing for Down Syndrome

Two types of pregnancy testing exist for Down syndrome: screening tests and prenatal diagnostic tests. Genetic counselors can give parents advice about which tests should be performed.

Screening tests give parents a risk assessment but do not predict conclusively if a child will have Down syndrome. They only determine if further testing is necessary.

Prenatal diagnostic tests tell parents if a fetus has the condition with 99 percent accuracy. However, these tests are invasive and can cause miscarriage.

Down Syndrome - Genetics and Down Syndrome

Several screening tests are available. They include:

  • Genetic ultrasound: Performed between 18 and 20 weeks, a detailed ultrasound measures certain fetal details that are correlated with Down syndrome.
  • Integrated screen: This uses results obtained from the first trimester screening tests and blood tests along with second trimester quad screen to obtain accurate results.
  • Nuchal translucency testing: Performed between 11 and 14 weeks, this test uses ultrasound to measure the clear space between the folds of tissue behind the baby’s neck.
  • Triple screen or quadruple screen (or multiple marker test): Performed between 15 and 18 weeks, it measures the quantities of genes related to Down syndrome in the maternal serum.

Diagnostic tests include:

  • Amniocentesis: Performed between 15 and 20 weeks, a small amount of amniotic fluid is sampled and tested for chromosomal abnormalities.
  • Chorionic villus sampling (CVS): Performed between eight and 12 weeks, a small piece of the placenta is sampled to obtain early accurate results.
  • Percutaneous umbilical blood sampling (PUBS): Performed after 20 weeks, a small amount of blood is collected from the umbilical cord and tested for genetic anomalies.

All three of these diagnostic tests are invasive and carry a risk for miscarriage.

Preparing for a Child with Down Syndrome

When parents learn they will be having a child with Down syndrome, they must prepare themselves so they do not feel overwhelmed. More than 90 percent of parents who find out they are having a child with Down syndrome choose to have an abortion.

Talking and visiting with other parents who have children with Down syndrome can help. Meeting a person with Down syndrome allows them to actually see what life could be like. Also, reading about the condition and staying informed can let the parents-to-be know what to expect. Enrolling children in early-intervention services can help speed along the developmental progress.

Resources

Harmon, Amy (2007). Prenatal test puts Down syndrome in hard focus. Retrieved September 21, 2008, from the New York Times Web site: http://www.nytimes.com/2007/05/09/us/09down.html?_r=2