Trisomy 21 — Down syndrome

March 21, Trisomy 21 Awareness Day

Trisomy 21 is the most well-known genetic condition. Named after John Langdon Down, the English physician who first published a description of the condition in 1866; Trisomy 21 is also known as ‘Down syndrome’. The condition was classified as a chromosomal condition in 1959 by the French physician Jerome Lejeune.

Trisomy 21 occurs when a person has 3 copies of chromosome 21 instead of the typical pair, making the total chromosomal count 47 instead of 46. The additional chromosomal copy can be passed during fertilization either from the maternal or the paternal gamete (reproductive cell). Less than 10% of Down syndrome cases are due to a paternal non-disjunction (uneven division of chromosomes) event1,2.  The rest are maternal, as increased maternal age increases the chances of non-disjunction events and subsequently, having a baby with Down syndrome1. The extra chromosome can be a whole chromosomal copy, a partial copy, or a mosaic trisomic form (some cells carry 3 chromosome 21 copies and other carry 2). Mosaicism is the least common form of Down syndrome, and partial forms due to translocation (chromosomes are arranged in the wrong order) are the second least common1. The latter is the only form of Down syndrome that can be inherited.

The additional chromosomal material affects the range of developmental and physical characteristics associated with Down syndrome. These may include – to different degrees – low muscle tone, small stature, upward slant to the eyes, and mild to moderate cognitive delays1,3,4. Children with Down syndrome reach developmental milestones later than typically developing children. Early intervention services, such as physical, occupational and speech therapies during the first years of life are imperative to optimize a child’s development3,4. Approximately 50% of people with Trisomy 21 are born with a heart defect; and have an increased risk for respiratory, hearing and vision problems, Alzheimer’s disease, thyroid conditions and childhood leukemia – but are less likely to develop solid tumor cancers3,5,6. Interestingly, some research shows that people with Down syndrome are also more likely to recover from heart defects and childhood leukemia than people without Down syndrome3.

In the US, 1 in 700 children is born with Down syndrome1. The estimated worldwide incidence is between 1 in 1,000 and 1 in 1,100 live births7. As Down syndrome is the most common genetic condition during pregnancy, there are multiple prenatal screening tests for pregnant women. These screening tests check for the chance of the fetus having a genetic condition. First and second trimester screening tests, that include biochemical testing, Nuchal Translucency (NT) measurement and ultrasounds have a 95% combined sensitivity, with a 5% False Positive Rate (FPR: reporting an unaffected pregnancy as affected)8. Non-Invasive Prenatal Testing (NIPT), which analyses genetic material from the pregnancy, has over 99% sensitivity, with an FPR of 0.001%9. All positive (affected) screening tests, including NIPT, should be confirmed with a diagnostic procedure like amniocentesis. Even though the chance of having a baby with Down syndrome – or any autosomal trisomy – increases with maternal age, most children with Down syndrome are born to women younger than 35 years old as birth rates are higher in younger women1. Consequently, prenatal screening of all pregnancies is recommended and should be offered, as prospective parents can gather useful information about the health of their baby, and arrange for any necessary clinical care as early as possible. The choice to undertake such screening and diagnostic tests, and all decisions regarding the pregnancy are individual ones that only the couple can take.

Trisomy 21 was recognized as ‘Mongolian Idiocy’ or ‘Mongolism’ before being renamed and accepted as ‘Down syndrome’ by the World Health Organization in 1965. During the first half of the twentieth century, children with Down syndrome were placed in institutions shortly after birth. The end of this practice, and the advancements in clinical care, saw the average lifespan of a person with Down syndrome rising from 25 to 60 years old3,6. Nowadays, there are special guidelines in place for treatment of newborns with disabilities and federal laws enforcing education for all children with disabilities. People with Down syndrome can receive graduate and postgraduate degrees, have jobs and live independently. The social, clinical and educational advancements were brought about by parent-driven groups who took action, spread awareness and demanded a level playing field for their children.

How each family takes on a Down syndrome diagnosis is a personal and intimate decision. The medical professionals’ role is to accurately and impartially provide the parents with all necessary information about the condition and the severity of each individual case, and society’s role is to be an inclusive and unbiased place for everyone.

NIPT results, possible next steps and clinical management should always be fully discussed with your healthcare provider. VERACITY and VERAgene both detect Trisomy 21, amongst other common genetic disorders, from the 10th week of pregnancy. To learn more please visit

For more information on support groups for trisomy 21 please visit: or

Popular Down Syndrome Books: The Memory Keeper’s Daughter (2005), We’ll paint the Octopus Red (1998)


  1. National Down Syndrome Society (2019),
  2. Hulten MA et al. (2010) ‘On the paternal origin of trisomy 21 Down syndrome’. Molecular Cytogenetics, 3:4
  3. Global Down Syndrome Foundation (2018),
  4. Genetics Home Reference (2019),
  5. Xavier AC and Taub JW (2010) ‘Acute Leukemia in children with Down syndrome’. Haematologica, 95:1043-1045
  6. Bergstrom S. et al. (2016) ‘Trends in Congenital Heart Defects in Infants With Down Syndrome’. Pediatrics, 138(1)
  7. World Health Organization (2019)
  8. Malone F et al. (2005) ‘First-trimester or second-trimester screening, or both, for Down’s syndrome. First and Second-Trimester Evaluation of Risk (FASTER) Research Consortium’. New England Journal of Medicine; 353(19):2001-2011.
  9. Mackie FL et al. (2017) ‘The accuracy of cell-free fetal DNA-based non-invasive prenatal testing in singleton pregnancies: a systematic review and bivariate meta-analysis’. BJOG, 2017;124:32-46.

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