Babies with trisomy are born with an extra copy of a specific chromosome in most or all of their cells. This means that they have three copies of this chromosome in each cell rather than the typical number, which is two. Health conditions that may be associated with trisomy include heart defects, vision or hearing problems, and intellectual and developmental disabilities.
Chromosomes are the structures in cells that contain genes. Each person normally has 23 pairs of chromosomes, or 46 in all. An individual inherits one chromosome from the mother’s egg and one from the father’s sperm. When an egg and sperm join together, they normally form a fertilized egg with 46 chromosomes.
Sometimes a mistake in cell division occurs before a woman gets pregnant. A developing egg or sperm ends up with an extra chromosome. When this cell joins with a normal egg or sperm cell, the resulting embryo has 47 chromosomes instead of 46.
Common trisomy conditions
Although trisomy can occur with any chromosome, here are the conditions that are most often associated with an extra chromosome:
- Trisomy 21 or Down syndrome: Down syndrome is one of the most common birth defects. In the US, about 6,000 babies (or 1 in 700) are born with Down syndrome each year. Most affected individuals have intellectual disabilities within the mild to moderate range. Although health conditions such as heart defects and vision and hearing problems are associated, most of these can be treated, and life expectancy is now about 60 years.
- Trisomy 18 is also called Edward syndrome: Trisomy 18 occurs in about 1 in 5,000 live births each year. Affected individuals may have heart defects, significant intellectual and developmental delays, and other life-threatening medical problems.
- Trisomy 13, also known as Patau syndrome: Trisomy 13 occurs in about 1 in 10,000 to 16,000 live births each year worldwide. Individuals with trisomy 13 often have heart defects, brain or spinal cord abnormalities, severe intellectual and developmental disabilities, and multiple physical problems in many parts of the body.
It is important to understand that every individual with a trisomy is unique and not all of them will have the same symptoms. The severity of the condition and the associated problems depend on:
- Which chromosome is duplicated: An extra copy of certain chromosomes, like chromosome 1, is not compatible with life and the embryo will not develop.
- How much of the extra chromosome is present: If only part of the chromosome is present, symptoms may be milder. If the complete chromosome is present, the symptoms may be more severe.
- How many cells have the extra chromosome: If the copy of the extra chromosome is in only a few cells (mosaicism), the symptoms are usually less severe than if all of the cells in the body are affected.
In the past 10 years, the March of Dimes has invested over 15 million dollars into research for chromosomal conditions, including trisomy. And many March of Dimes grantees are studying basic biological processes of development. This important research should improve our understanding of how genes and other factors affect the development of a baby.