Down syndrome is a genetic disorder caused by an additional number 21 chromosomes. There are three ways this can happen:

Trisomy 21

Trisomy 21 is the most common type of Down syndrome, and this happens in about 95% of all individuals with Down syndrome. It occurs when there are three, rather than two, number 21 chromosomes present in every cell of the body. Instead of the usual 46 chromosomes, a person with Trisomy 21 Down syndrome has 47 chromosomes.


Translocation accounts for 4% of all cases of Down syndrome. In translocation, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14. While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.


Mosaicism occurs when nondisjunction of chromosome 21 takes place in one – but not all – of the initial cell divisions after fertilization. When this occurs, there is a mixture of two types of cells, some containing the usual 46 chromosomes and others containing 47. Mosaicism accounts for about 1% of all cases of Down syndrome.

Down syndrome: Additional chromosomal material

It is this additional genetic material that alters the course of development and causes the characteristics associated with the syndrome. Trisomy 21 accounts for 95% of cases.

Who is at risk?

Advanced maternal age is a risk factor for Down syndrome. Being over 35 at the time of pregnancy increases the risk of having a child with Down syndrome to one in 400. That risk jumps to one in 35 for women over 45. 

Despite the increased risk to women over 35, women under the age of 35 give birth to more children with Down syndrome than older women. Statistically, the increased number of Down syndrome cases is associated with the number of overall births to women under 35 – more births equates to more children born with the syndrome. Women who have given birth to a child with Down syndrome are at increased risk of having a second or subsequent child with the condition. 


The duplicate chromosome 21 causes physical and mental abnormalities of varying severity. Physical abnormalities may affect the head, eyes, neck, and fingers.

Children with Down syndrome tend to have flat facial features, upward slanted eyes (not related to ethnicity), short neck and fingers and smaller than average head size. Patients may also develop poor muscle tone. 

Physical and mental development may be slower than peers or stunted. Mental and social delays are typically present – though in varying degrees. Delays may manifest as:

  • Slower learning
  • Agitation
  • Shortened attention span

Diagnosing Down syndrome

Physical abnormalities may affect the physical development of internal organs, including the brain, eyes, heart and other organs. Down syndrome may increase the likelihood of:

  • Heart-related birth defects
  • Dementia
  • Vision problems
  • Gastrointestinal blockage
  • Joint problems
  • Chronic constipation
  • Hypothyroidism


There is no treatment or cure for Down syndrome. Patient health issues are treated on a case-by-case basis. Patients with some organ deformities, including heart defects or gastrointestinal blockage may require surgery.

Down syndrome and pregnancy

Down syndrome does not typically affect a woman's ability to get pregnant, stay pregnant or give birth to a normal, healthy baby. Women with Down syndrome may be at increased risk of sexual abuse.

Life expectancy

Patients with Down syndrome can live long, productive lives with proper medical care, special education, and life training. Medical attention may be required for life due to the potential for physical abnormalities and conditions.

Read More:
Down Syndrome Risk Calculator
Down Syndrome Ultrasound Markers
Early Screening: Down Syndrome