Down Syndrome

Also known as trisomy 21

Down syndrome, also known as trisomy 21, is a genetic disorder that occurs when a person has a full or partial extra copy of chromosome 21. It's characterized by a variety of distinctive physical features, an increased risk of certain medical problems, and varying degrees of developmental and intellectual delays. Trisomy 21 is an umbrella term for three different types of chromosome 21 abnormalities: complete trisomy 21, translocation trisomy 21, and mosaic trisomy 21. 

Approximately 6,000 babies in the United States are born with the syndrome each year.﻿﻿ In order for a child with Down syndrome to reach their full potential, early intervention is key. As a parent, the more you learn about Down syndrome, the better equipped you will be to set your child up for success.

Frequently Asked Questions

  • What causes Down syndrome?

    Typically, babies inherit 23 pairs of chromosomes (for a total of 46) from their parents, but in cases of Down syndrome, they get an extra full or partial copy of chromosome 21, so they'll have 47. We know this is usually a result of the twenty-first pair of chromosomes from either the egg or the sperm failing to separate, but the cause of this phenomenon is still unknown.﻿﻿﻿

  • Is Down syndrome genetic?

    All three types of Down syndrome are genetic conditions, but only one type, translocation trisomy 21, is hereditary (meaning passed from parent to child through the parents' genes), accounting for just 1% of all cases of Down syndrome.﻿﻿ The condition occurs in people of all races and ethnicities, though the likelihood increases if the mother is above age 35.

  • How is Down syndrome diagnosed?

    During pregnancy, Down syndrome can be diagnosed (or at least suspected) through imaging tests, such as specialized ultrasounds; via maternal blood tests, such as a quadruple screen; through cell-free DNA tests; and via amniocentesis or chorionic villi sampling. Once the baby is born, Down syndrome can be diagnosed almost immediately, due to the defining physical characteristics of the condition.

  • Can people with Down syndrome have kids?

    Many women with Down syndrome are able to have children, though Down syndrome is associated with early menopause. A few older studies have concluded that many males with Down syndrome may have infertility, but these studies are being reinvestigated.﻿﻿

  • How common is down syndrome?

    Down syndrome is the most common chromosomal condition in the United States, affecting 1 of every 700 births.﻿﻿ As maternal age increases, the chance of having a baby born with Down syndrome increases. At maternal age 35, the chance is 1 in 350. At age 40, it's 1 in 100. At age 45, it's 1 in 30. That said, 80% of babies with the condition were born to mothers under 35.﻿﻿

Key Terms

Fluorescence light micrograph of the 46 chromosomes from a normal human female. This constitutes the full karyotype, that is the total number of chromosomes found in nearly every cell of the female human body. Each cell contains 22 matched pairs of chromosomes, and one sex-determining pair (bottom right). Female and male karyotypes differ only in the sex pair: male sets would be labeled XY instead of the X pair here.
Reasons Why Your Doctor May Order a Karyotype
Child with Down syndrome
Down Syndrome Increases the Chance of Developing Thyroid Disease
Child with Down syndrome
Seizures Often Seen in Down Syndrome Can Happen to Children or Adults
Elderly doctor looking at x-rays
Treating Atlantoaxial Instability in Down Syndrome
Page Sources
  1. National Down Syndrome Society. What is Down syndrome?.

  2. National Down Syndrome Society. Does Down syndrome run in families?

  3. Parizot E, Dard R, Janel N, Vialard F. Down syndrome and infertility: what support should we provide?. Journal of Assisted Reproduction and Genetics. 2019 Jun 15; 36(6): 1063-1067. doi:10.1007/s10815-019-01457-2

  4. Centers for Disease Control and Prevention. Facts about Down Syndrome. Updated December 2019.

  5. National Down Syndrome Society. Down syndrome facts.

  6. American Association on Intellectual and Developmental Disabilities. FAQs on intellectual disability.

Additional Reading
  • National Human Genome Research Institute. Chromosome fact sheet. Updated August 2020.

  • National Human Genome Research Institute. Genetic disorders. Updated May 18, 2018.

  • Scott JG, Mihalopoulos C, Erskine HE, et al. Childhood mental and developmental disorders. In: Patel V, Chisholm D, Dua T, et al., editors. Mental, Neurological, and Substance Use Disorders: Disease Control Priorities, Third Edition (Volume 4). Washington (DC): The International Bank for Reconstruction and Development / The World Bank; 2016 Mar 14. Chapter 8.