Genetics Northern California

Chromosome abnormalities

Chromosome Abnormalities


Chromosomes are the packages of genetic information that we inherit from our parents and pass on to our children. Chromosomes are found in every cell of the body and contain the genes, which are responsible for growth and development. Usually, healthy people have 46 chromosomes in every cell, 23 pairs.


A chromosome abnormality happens when a baby is born with some change in the number or structure of the chromosomes. This leads to the individual having extra or missing chromosomal material, known as a chromosome abnormality. Many chromosome abnormalities cause birth defects and mental retardation.


Down syndrome is the most common chromosome abnormality. About 1 out of every 800 babies is born with this condition. Down syndrome is also known as trisomy 21. It occurs when there is an extra or third copy of chromosome #21. Features of Down syndrome include moderate mental retardation and a typical facial appearance. About 40% of individuals with Down syndrome also have heart defects. Other less common chromosome abnormalities may be more or less severe than Down syndrome.


Trisomy 18 and trisomy 13 are two other chromosome abnormalities. About 1 in every 4000 babies is born with one of these conditions.  It happens when a baby has an extra (or third) copy of either chromosome number 18 or 13 in all of the cells in the body. The extra copy of chromosome 18 or 13 is present from the time of conception. Babies with either trisomy 18 or trisomy 13 often have many birth defects including heart, brain, and kidney abnormalities. Babies born with one of these chromosome abnormalities seldom live longer than a few days or weeks. Many affected preganancies end in miscarriage or stillbirth. 


Chance of Having a Baby with a Chromosome Abnormality

Most chromosome abnormalities (including most cases of Down syndrome) are not hereditary, that is, they do not run in families. Older women have a greater risk than younger women for having a baby with a chromosome abnormality. Even though the risk gradually increases with age, a woman of any age can have a baby with a chromosome abnormality.


The following chart shows the approximate chances of having a baby with a chromosome abnormality: Maternal Age Risks