Down syndrome: What you need to know

March 21, 2019

World Down Syndrome Day is observed every year on March 21. According to the Centers for Disease Control and Prevention (CDC), Down syndrome (also called trisomy 21) affects about 1 in every 700 babies born in the United States each year.

What is Down syndrome?

Down syndrome is the most common chromosomal condition in this country. A chromosomal condition happens when there’s a problem in one or more chromosomes. Chromosomes are the structures that hold genes. And genes are part of your body’s cells that store instructions for the way your body grows and works. People with Down syndrome may have physical problems, learning problems and distinct physical features.

What causes Down syndrome?

Each person has 23 pairs of chromosomes in every cell in their body. For each pair, you get one chromosome from your mother and one from your father. Babies with Down syndrome have an extra copy of chromosome 21. This extra copy changes the way the brain and body develop.

We don’t know for sure what causes the extra copy of chromosome 21. It may be that the egg or sperm that come together in fertilization has an extra chromosome.

One thing to note is that your age when you get pregnant affects your risk of having a baby with Down syndrome. The older you are, the higher your risk. Learn more here.

What are the different kinds of Down syndrome?

There are three kinds of Down syndrome:

  1. Trisomy 21. This is the most common kind. About 95 percent of people with Down syndrome have trisomy 21. In this condition, there are three copies of chromosome 21 instead of two.
  2. Mosaic Down syndrome. People with this condition have some cells with three copies of chromosome 21, but other cells have the usual two copies. Mosaic Down syndrome is rare.
  3. Translocation Down syndrome. This condition happens when a person has two copies of chromosome 21, plus an extra part of a chromosome or a whole extra chromosome that’s attached (also called translocated) to one copy of chromosome 21. This condition is rare, and it can be inherited (passed from parent to child).

Health care providers offer testing for Down syndrome to all pregnant women as part of regular prenatal care. You can decide if you want to have these tests. Your provider or a genetic counselor can guide you and your partner about testing. If you or your partner has a family history of Down syndrome, be sure to tell your provider or genetic counselor.