Also known as trisomy 21
Diana Apetauerova, MD, is board-certified in neurology with a subspecialty in movement disorders. She is an associate clinical professor of neurology at Tufts School of Medicine.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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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
Centers for Disease Control and Prevention. Facts about Down Syndrome. Updated December 2019.
National Down Syndrome Society. Down syndrome facts.
American Association on Intellectual and Developmental Disabilities. FAQs on intellectual disability.
National Human Genome Research Institute. Chromosome fact sheet. Updated August 2020.
National Human Genome Research Institute. Genetic disorders. Updated May 18, 2018.
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