Birth defects

• Birth defects can cause disabilities and even death.
• Alcohol and drug use while pregnant can cause birth defects.
• Ask your provider what you can do to prevent birth defects.

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Babies with chromosomal conditions have a problem in one or more of their chromosomes. Chromosomes are the structures that hold genes. Genes are part of your body's cells that store instructions for the way your body grows and works. Genes are passed from parents to children.

Each person has 23 pairs of chromosomes, or 46 in all. For each pair, you get one chromosome from your mother and one chromosome from your father.

About 1 in 150 babies is born with a chromosomal condition. Down syndrome is an example of a chromosomal condition. Because chromosomes and genes are so closely related, chromosomal conditions are also called genetic conditions.

What causes chromosomal conditions?
Chromosomal conditions are caused by two kinds of changes in chromosomes: 

1. Changes in the number of chromosomes—This means you have too many or too few chromosomes.
2. Changes in the structure of chromosomes—This means that part of a chromosome may be missing, repeated or rearranged.

Both kinds of changes can be inherited. This means they’re passed from parent to child. Or they can happen randomly as cells develop.

What problems can chromosomal conditions cause?
Sometimes chromosomal conditions can cause miscarriage. This is when a baby dies in the womb before 20 weeks of pregnancy. More than half of miscarriages are caused by chromosomal conditions. These conditions also can cause stillbirth, which is when a baby dies in the womb before birth but after 20 weeks of pregnancy.

Each child born with a chromosomal condition is different. Some children with chromosomal conditions have intellectual disabilities or birth defects, or both. Some children with these conditions don’t have any serious problems. The problems depend on which chromosomes are affected and how.

How do you know if your baby has a chromosomal condition?
The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women be offered prenatal tests for Down syndrome and other chromosomal conditions. A screening test is a medical test to see if you or your baby is more likely than others to have a certain health condition.

You can have screening tests in the first or second trimester of pregnancy. First trimester screening is done at 11 to 13 weeks of pregnancy. Along with a blood test, you get a special ultrasound that checks the back of your baby’s neck. Testing in the second trimester is called maternal blood screening. You can get this blood test between 15 and 20 weeks of pregnancy.

If a screening test shows that your baby may have a problem, your provider gives you a diagnostic test. This is a medical test to see if you do or don't have a certain health condition. Diagnostic tests include amniocentesis or chorionic villus sampling. Your provider also can check your baby’s blood for chromosomal conditions after he’s born.

What are the chances of your baby having a chromosomal condition?
As you get older, there’s a greater chance of having a baby with certain chromosomal conditions, like Down syndrome. For example, at age 35, your chances of having a baby with a chromosomal condition are 1 in 192. At age 40, your chances are 1 in 66.

If you or someone in your family has a chromosomal condition, or if you have a baby with a chromosomal condition, talk to a genetic counselor. A genetic counselor is a person who is trained to know about genetics, birth defects and other medical problems that run in families. She can help you understand the causes of chromosomal conditions, what kind of testing is available, and your chances of having a baby with these conditions. If you already have a baby with a chromosomal condition, the chances of having another baby with the same condition are usually low.

For more information
Chromosome Disorder Outreach
Genetic Alliance
National Down Syndrome Society
National Organization for Rare Disorders

Last reviewed February 2013

See also: Birth defects, Genetic counseling, Your family health history