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Down syndrome Facts

What Is Down syndrome?

Down syndrome is genetic condition, which causes delays in physical, cognitive and language development due to the presence of an extra chromosome on the 21st pair. People with Down syndrome have 47 chromosomes instead of the usual 46. The degree of development delay affecting the individual varies. Like all people, individuals with Down syndrome thrive in the home, school environment, workplace and community when afforded the full spectrum of life opportunities and experiences.

Down syndrome occurs in approximately one in every 733 live births and affects people of all ages, races and socioeconomic levels. Each year approximately 5,000 to 6,000 babies are born with Down syndrome and it is estimated that there more than 400,000 people with Down syndrome living in the United States.

At present, it is not known what causes Down syndrome. The condition results from an error in cell division call non-disjunction. Down syndrome usually occurs during the development of the sperm or the egg before conception, or immediately following conception. The extra chromosome can originate in either the mother or father, but more often in the mother. The risk of having a child with Down syndrome increases with advancing age of the mother. However, most babies with Down syndrome are born to mothers under the age of 35 years. It has been reported that the age of the father may also be a contributing factor in the occurrence of Down syndrome. It is essential for both parents to understand that they are not personally responsible for the extra chromosome, or that their baby has Down syndrome. Nothing in the parent’s diet, activity, emotional state or past experiences caused this genetic anomaly. 

Chromosome Subtypes

A normal set of chromosomes has 22 evenly paired chromosomes plus the sex chromosomes. In the illustration below, the XX means that this person is a female. The test in which blood or skin samples are checked for the number and type of chromosomes is called a karyotype.           

Normal Set of Chromosomes

There are three main types of chromosome variations in Down syndrome:

Trisomy 21: The majority of people with Down syndrome, approximately 95%, have an extra 21 chromosome. Instead of the normal number of 46 chromosomes in each cell, individuals with Trisomy 21 syndrome have 47. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate (non-disjunction). As the embryo develops, the extra chromosome is replicated in every cell of the body.

Trisomy 21 

Translocation: The extra 21 chromosome is attached or translocated on to another chromosome, usually on chromosome 14, 21, or 22. When translocation is determined, a chromosomal analysis of the parents is recommended. Translocation occurs in approximately 3%-4% of people with Down syndrome.


Mosaicism: This variation occurs in approximately 1%-2% of the people with Down syndrome. Some cells have 47 chromosomes and others have 46. As a result, the individual may have fewer of the usual physical characteristics. How the baby is affected depends on where these cells are in the body. At the present time, there is not much research on the similarities and differences between simple trisomy 21 and mosaic trisomy 21. 

How Is The Diagnosis Made?

A baby with Down syndrome is typically identified at birth by the presence of certain physical traits. The most common characteristics include low muscle tone, a slightly flattened facial profile and upward slant to the eyes. Other characteristics include low birth size and weight and possibly heart murmurs. Because these characteristics may be present in babies without Down syndrome, a chromosomal analysis is necessary in order to confirm the clinical diagnosis. A blood sample is taken from the baby to perform this test.  

Prenatal Diagnosis

If you have recently received a prenatal diagnosis that your baby has Down syndrome, you may feel overwhelmed by this information and inadequately prepared for the future of your child. The Down Syndrome Association of San Diego offers emotional support, information and resources. The All Children Are Special Family Resource Guide is an invaluable source of information providing direction and current information. 

Family Resource Guide

All Children Are Special was designed to assist parents in gaining insight and knowledge about Down syndrome. You will soon discover that your baby… like all babies… is unique. Down syndrome is just one aspect of your child’s whole being. This Family Resource Guide provides current information about Down syndrome and resources, both local and national. Additionally, at your request, we can arrange for a “support parent” to contact you. Almost without exception, parents will agree that their primary source of information and support has come from sharing their experience with one another. No one else will understand your emotions better. View the All Children Are Special Family Resource Guide. This guide is also available in Spanish. Vea Todos los niños son especiales guía de recursos para familias. 

Speaking about Down syndrome

People with Down syndrome should always be referred to as people first. Instead of “a Down syndrome child,” it should be “a child with Down syndrome.” Also avoid “Down’s child” and describing the condition as “Down’s,” as in, “He has Down’s.” Down syndrome is a condition or a syndrome, not a disease. People “have” Down syndrome, they do not “suffer from” it and are not “afflicted by” it. While it is unfortunately clinically acceptable to say “mental retardation,” you should use the more socially acceptable term “intellectual disability”. The Down Syndrome Association of San Diego and The National Down Syndrome Society strongly condemn the use of the word "retarded" in any derogatory context. Using this word is hurtful and suggests that people with disabilities are not competent. 

The preferred spelling is Down syndrome, rather than Down’s syndrome. Many dictionaries contain both spellings (with or without an apostrophe s), but the preferred usage in the United States is Down syndrome. This is because an “apostrophe s” connotes ownership or possession. Down syndrome is named for the English physician, John Langdon Down, who characterized the condition, but did not have it. The AP Stylebook recommends using “Down syndrome,” as well. 



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