What causes Down syndrome?

Down syndrome is caused by a random error in cell division that results in the presence of an extra copy of chromosome 21.

The type of error is called nondisjunction. Usually when one cell divides in two, pairs of chromosomes are split so that one of the pair goes to one cell and the other from the pair goes to the other cell. In nondisjunction, something goes wrong and both chromosomes from one pair go into one cell and no chromosomes from that pair go into the other cell.

Most of the time, the error occurs at random during the formation of an egg or sperm. To date, no behavioral activity of the parents or environmental factor is known to cause Down syndrome.1,2

After much research on these cell division errors, researchers know that2:

  • In the majority of cases, the extra copy of chromosome 21 comes from the mother through the egg.
  • In a small percentage (less than 5%) of cases, the extra copy of chromosome 21 comes from the father through the sperm.
  • In the remaining cases, the error occurs after fertilization, as the embryo grows.

NICHD launched DS-Connect® as a safe and secure online registry for people with Down syndrome, their families, and researchers to share information and participate in clinical trials or research. As the registry grows, families and researchers learn more about Down syndrome and identify similarities and differences in the symptoms and treatment of people with Down syndrome from around the world. The information collected helps guide future research and treatment. Learn more about DS-Connect®: The Down Syndrome Registry.

Chromosomal Changes That Can Cause Down Syndrome

Research shows that three types of chromosomal changes can lead to Down syndrome.

  • Complete trisomy 21. In this case, an error during the formation of the egg or the sperm results in either one having an extra chromosome. So after the egg and sperm unite, the resulting cells will also have three copies of chromosome 21. The complete extra copy of chromosome 21 is in all of the person’s cells—or a complete trisomy. Almost all Down syndrome cases result from complete trisomy 21.1,2,3
  • Mosaic trisomy 21. Not every cell in the body is exactly the same. In a small percentage (less than 5%) of Down syndrome cases, most of the cells in the body have the extra chromosome, but some of them don’t. This is called “mosaicism.” Mosaic trisomy 21 can occur when the error in cell division takes place early in development but after a normal egg and sperm unite. It can also occur early in development when some cells lose an extra chromosome 21 that was present at conception. The symptoms of someone with mosaic trisomy 21 may vary from those of someone with complete trisomy 21 or translocation trisomy 21, depending on how many cells have the extra chromosome.1,2
  • Translocation trisomy 21. In this type of chromosomal change, only part of an extra copy of chromosome 21 is in the cells. The extra part of the chromosome gets “stuck” to another chromosome and gets transmitted into other cells as the cells divide. This type of change causes a small number of Down syndrome cases. There are no distinct cognitive or medical differences between people with translocation trisomy 21 and those with complete trisomy 21.

Sometimes, a parent who does not have Down syndrome may carry a translocation in chromosome 21 that can be passed on to children and cause Down syndrome. Studying the parents’ chromosomes can reveal whether this is the cause of the syndrome. A genetic counselor can assist families affected by translocation trisomy 21 in understanding the risk of Down syndrome in future pregnancies.1,2,3


  1. NDSS. (n.d.). About Down syndrome. Retrieved June 11, 2012, from https://ndss.org/about external link
  2. Parker, S. E., Mai, C. T., Canfield, M. A., Rickard, R., Wang, Y., Meyer, R. E., et al. (2010). Updated national birth prevalence estimates for selected birth defects in the United States, 2004–2006. Special Issue: 2010 Congenital Malformation Surveillance Report: A Report from the National Birth Defects Prevention Network. Clinical and Molecular Teratology, 88(12), 1008–1016.
  3. Genetic and Rare Diseases Information Center (GARD). (2012). Down syndrome. Retrieved June 11, 2012, from http://rarediseases.info.nih.gov/GARD/QnASelected.aspx?diseaseID=10247
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