Down Syndrome

Also known as trisomy 21

Medically reviewed by Diana Apetauerova, MD
Updated on March 09, 2023

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.

Understanding Down Syndrome

toddler with down syndrome with mom
Health Challenges of Down Syndrome
happy boy with down syndrome
Facts About Down Syndrome
Parent in kitchen holding child who has Down syndrome
What to Know About Down Syndrome
happy boy with down syndrome
Common Characteristics of Down Syndrome
girl with down syndrome in occupational therapy
Treating Conditions Caused By Down Syndrome

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.

    Learn More Causes of Down Syndrome
  • 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.

    Learn More Causes of Down Syndrome
  • 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.

    Learn More How Down Syndrome Is Diagnosed
  • 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.

Diagnosis

pregnant woman with ultrasound pictures
How Down Syndrome Is Diagnosed
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 Healthcare Provider May Order a Karyotype
Human Karyotype
The Purpose and Steps Involved in a Karyotype Test

Key Terms

  • Chromosome

    A threadlike structure containing genetic material located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of nucleic acid, known as DNA. When changes occur in the number of chromosomes, it can lead to major alterations, as in the case of trisomy 21.

  • Congenital Disorder

    The term congenital means "present before or at birth." Congenital disorders may affect physical development, function, or appearance, and they may be a result of environmental, genetic, or a combination of both factors. Down syndrome is a congenital condition caused by genetic factors.

    Congenital Conditions and Diseases
  • Developmental Disorder

    An umbrella term for various developmental delays that may arise in the areas of language, speech, motor coordination, or scholastic skills in children. If a developmental delay seems present in a child with Down syndrome, it may be helpful to have a psychologist administer a developmental screening.

  • Genetic Disorder

    A condition caused by a mutation (change) in a person's DNA sequence. Down syndrome is considered a genetic disorder because it results from a change in chromosomes, the structures that carry genes.

  • Intellectual Disability

    A disability affecting intellectual reasoning and cognitive functioning that sets in before age 18. It may affect social skills and practical skills, but special education and behavioral therapy can be helpful tools.

Page 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. 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.