Chromosome Disorders

Genetic birth defects can be caused by errors in an entire chromosome or part of a chromosome, rather than in a single gene. These errors occur when the egg or sperm are forming. Occasionally, an extra chromosome is present or a chromosome is missing or broken.

This type of problem can come from either the mother or the father. The resulting imbalance of genetic material gives the fertilized egg too little or too much genetic information.

When such an error happens, it can be perpetuated each time the cells go through the division process. As the embryo grows, the additional or missing genetic information can translate into a wide range of abnormal body structures and functions, such as heart or kidney conditions and, often, mental retardation.

The chances of having a child with a chromosomal inheritance error become greater as women grow older. The possibility of such an error—commonly resulting in Down syndrome—increases throughout the childbearing years. Down syndrome is a pattern of mental retardation and physical abnormalities, often including heart defects.

Down Syndrome Occurrence and Mother’s Age
- Women in their 20s who become pregnant have about a one-in-1,230 chance of having a pregnancy affected with Down syndrome.
- At age 30, it’s one in 690.
- By age 35, the chances increase to one in 270.
- At age 40, the risk in one in 78.
- At age 45, chances are one in 22.

Prenatal tests such as chorionic villus sampling or amniocentesis can help rule out the presence of a chromosomal error with a high degree of certainty in pregnancy.

Parents who have had a baby with a chromosomal abnormality should consult a genetic counselor. These health professionals help families understand what is known about the causes of a birth defect and the chances that it will recur in another pregnancy. Fortunately, in most cases, parents of a baby with a chromosomal abnormality will learn that the risk is low. For examples, parents of a baby with Down syndrome usually have a 1 percent risk of having another affected baby, plus whatever additional risk a mother has, based on her age.