Causes of Down Syndrome

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Down syndrome is caused when one's genetic code has an extra copy of chromosome 21 (or part of one). Since chromosomes normally come in pairs, this is often referred to as trisomy 21.  

It's not always clear why this anomaly occurs. In most cases, it's a random occurrence at the time a sperm fertilizes an egg, although certain risk factors for Down syndrome have been identified, and there is one type of the disorder that can be inherited.

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Trisomy for a specific chromosome, including for chromosome 21, is the result of a misdivision in the sperm or the egg prior to conception. Each of the three types of trisomy 21 presents slight nuance in terms of exactly how it is caused:

  • Complete trisomy 21: Chromosomes line up to divide and create eggs or sperm in a process called meiosis. With this type of Down syndrome, non-disjunction occurs. That is, an egg is gifted with two 21st chromosomes, rather than one. Once fertilized, that egg then has a total of three chromosomes. This is the most common way Down syndrome occurs.
  • Translocation trisomy 21: In a translocation, there are two copies of chromosome 21, but extra material from a third 21st chromosome is attached (translocated to) another chromosome. This type of Down syndrome may occur either before or after conception and is the form that may sometimes be passed down (inherited). 
  • Mosaic trisomy 21: This is the least common form of Down syndrome. It occurs after conception for unknown reasons and differs from the other two types of trisomy 21 in that only some cells have an extra copy of chromosome 21. For this reason, the characteristics of someone with mosaic Down syndrome aren't as predictable as those of complete and translocation trisomy 21. They may seem less obvious depending on which cells and how many cells have a third chromosome 21.

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Only one type of Down syndrome—translocation Down syndrome—is considered inheritable. This type is very rare. Of those, only a third are thought to have inherited the translocation.

A translocation that will ultimately lead to a child having Down syndrome often takes place when the parent of that child is conceived. Part of one chromosome breaks off and becomes attached to another chromosome during cell division. This process results in three copies of chromosome 21, with one copy attached to another chromosome, often chromosome 14.

This anomaly will not affect the normal development and function of the parent because all the genetic material required on the 21st chromosome is present. This is called a balanced translocation. When someone with a balanced translocation conceives a child, though, there's a chance that this will cause that child to have an extra chromosome 21 and therefore be diagnosed with Down syndrome.

There's an increased likelihood that the parents of a child with Down syndrome due to a translocation will have other children with the disorder. It also is important that parents of a child with translocation know that their other children could be carriers and could be at risk of having a baby with Down syndrome in the future.

If a woman with Down syndrome becomes pregnant, she's at an increased risk of having a baby with Down syndrome but she's just as likely to have a child who doesn't have the disorder.

Most published data suggest that 15% to 30% of women with Down syndrome are capable of becoming pregnant, and their risk of having a child with Down syndrome is approximately 50%.

Risk Factors

There are no environmental factors such as toxins or carcinogens that can cause Down syndrome, nor do lifestyle choices (such as drinking, smoking, or taking drugs) play a part. The only known non-genetic risk factor for having a child with Down syndrome is what is sometimes referred to as advanced maternal age (being over 35).

This doesn't mean, however, that having a baby before age 35 is a reliable strategy for preventing Down syndrome. Roughly 80% of children with Down syndrome are born to women who are younger than 35.

Here is how the risk of Down syndrome increases with maternal age:

Age Risk
25 1 in 1,250
30 1 in 900
35 1 in 350
40 1 in 100
45 1 in 30
49 1 in 10

Some women over the age of 35 (or with other risk factors) may choose to have prenatal testing, such as amniocentesis, to screen for Down syndrome. The American College of Obstetricians and Gynecologists recommends, in fact, that all women be offered these options.

The decision of whether or not to do this is very personal and should be made with the assistance of genetic counseling. Such tests do pose notable risks that are worth knowing more about before you make your decision.

Frequently Asked Questions

  • What are the chances of having a baby with Down syndrome?

    Roughly one in 700. According to statistics, one in 700—or approximately 6,000—babies are born with Down syndrome in the United States each year. 

  • What are risk factors for Down syndrome?

    The top risk factor for having a child with Down syndrome is advanced maternal age. That means the mother is 35 years or older at the time of the due date. At age 25, the risk of Down syndrome is one in 1,250. At 35, the risk increases to one in 350. At 40, it is one in 100. By age 49, the risk is one in 10.

  • Is Down syndrome genetic?

    Yes and no. Down syndrome is caused by an extra copy of a specific chromosome. In most cases, this is the result of something going wrong with the sperm or the egg prior to conception. However, in some cases, Down syndrome is genetic. Translocation trisomy 21, an inheritable condition, makes up about 4% of Down syndrome cases. 

  • Can folic acid prevent Down syndrome?

    Folic acid is an important B vitamin that is taken during pregnancy to prevent certain birth defects, such as spina bifida. It is unclear whether folic acid can prevent Down syndrome. Some studies suggest folic acid may help to prevent trisomy 21 in babies born to older mothers, but the data is inconclusive. 

10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. March of Dimes. Down syndrome.

  2. National Down Syndrome Society. What is Down syndrome?

  3. National Down Syndrome Society. Does Down Syndrome Run in Families?

  4. American Pregnancy Association. Down Syndrome: Trisomy 21.

  5. Centers for Disease Control and Prevention. Facts about Down Syndrome.

  6. National Down Syndrome Society. Down syndrome facts.

  7. Practice Bulletin No. 162: Prenatal Diagnostic Testing for Genetic Disorders. Obstet Gynecol. 2016;127(5):e108-22. doi:10.1097/AOG.0000000000001405

  8. National Down Syndrome Society. Down syndrome.

  9. Centers for Disease Control and Prevention. Folic acid helps prevent some birth defects.

  10. Hollis ND, Allen EG, Oliver TR, et al. Preconception folic acid supplementation and risk for chromosome 21 nondisjunction: a report from the National Down Syndrome Project. Am J Med Genet A. 2013;161A(3):438-44. doi:10.1002/ajmg.a.35796

By Vincent Iannelli, MD
 Vincent Iannelli, MD, is a board-certified pediatrician and fellow of the American Academy of Pediatrics. Dr. Iannelli has cared for children for more than 20 years.