There can be impairments in cognitive ability and physical growth, mild to moderate developmental disabilities, and a higher risk of some health problems.
Through a series of screenings and tests, Down syndrome can be detected before or after birth.
The likelihood having Down syndrome is around 1 in every 700 pregnancies. It is determined by many factors, but research suggests there is a higher risk if the mother delivers at over 35 years of age.
Before the age of 30 years, fewer than one in 1,000 pregnancies will be affected by Down syndrome. After the age of 40 years, this figure rises to about 12 in 1,000.
Here are some key points about Down syndrome. More detail is in the main article.
- Older women are more likely to give birth to a child with Down syndrome.
- Normally there are two copies of every chromosome. In Down syndrome, there are three copies, either complete or partial, of chromosome 21.
- The characteristics of Down syndrome include low muscle tone, short stature, flat nasal bridge, and a protruding tongue.
- Individuals with Down syndrome have a higher risk of some diseases including Alzheimer's disease and epilepsy.
- Screening tests can be used during pregnancy to estimate the probability that a child will have Down syndrome.
What is Down syndrome?
Down syndrome, also known as trisomy 21, is a genetic disorder.
Down syndrome is a genetic condition that results when there is an extra copy of a specific chromosome, chromosome 21.
It is not an illness but a term that describes the features resulting from this change.
The extra chromosome can affect the physical features, intellect, and overall development of an individual.
It also increases the likelihood of some health problems.
Down syndrome happens when there is an extra copy of genetic material on all or part of the 21st chromosome.
Every cell in the body contains genes that are grouped along chromosomes in the cell's nucleus. There are normally 46 chromosomes in each cell, 23 inherited from the mother and 23 from the father.
When some or all of a person's cells have an extra full, or partial, copy of chromosome 21, the result is Down syndrome.
People with Down syndrome often have distinct physical features, unique health issues, and variability in cognitive development.
Physical characteristics include:
- eyes that have an upward slant, oblique fissures, epicanthic skin folds on the inner corner, and white spots on the iris
- low muscle tone
- small stature and short neck
- flat nasal bridge
- single, deep creases across the center of the palm
- protruding tongue
- large space between large and second toe
- a single flexion furrow of the fifth finger
Individuals with Down syndrome usually have cognitive development profiles that suggest mild to moderate intellectual disability. However, cognitive development and intellectual ability are highly variable.
Children with Down syndrome often reach developmental milestones later than their peers.
There may be a delay in acquiring speech. A child may need speech therapy to help them gain expressive language.
Fine motor skills may also be delayed. They can take time to develop after gross motor skills have been acquired.
On average, a child with Down syndrome will:
- sit at 11 months
- crawl at 17 months
- walk at 26 months
There may also be problems with attention, a tendency to make poor judgments, and impulsive behavior.
However, people with Down syndrome can attend school and become active, working members of the community.
There may also be a higher risk of:
- respiratory problems
- hearing difficulties
- Alzheimer's disease
- childhood leukemia
- thyroid conditions
Women with a higher chance of having a child with Down syndrome may receive screening and diagnostic tests.
Screening tests can estimate the probability of Down syndrome being present. Some diagnostic tests can definitively tell whether the fetus has the condition.
Women aged 30 to 35 years or older may receive genetic screening during pregnancy because the risk of having a child with Down syndrome increases as women age.
Screening tests include:
- Nuchal translucency testing: At 11 to 14 weeks, an ultrasound can measure the clear space in folds of tissue behind the neck of a developing baby.
- Triple screen or quadruple screen: At 15 to 18 weeks, this measures the quantities of various substances in the mother's blood.
- Integrated screen: This combines the results of first-trimester blood tests and screening tests, with or without nuchal translucency, with second-trimester quadruple screening results.
- Cell-free DNA: A blood test that analyzes fetal DNA found in the maternal blood.
- Genetic ultrasound: At 18 to 20 weeks, a detailed ultrasound is combined with blood test results.
Screening is a cost-effective and less invasive way to determine if more invasive diagnostic tests are needed. However, unlike diagnostic tests, they cannot confirm whether Down syndrome is present.
Diagnostic tests are more accurate in detecting Down syndrome and other problems. They are usually performed inside the uterus, and they increase the risk of miscarriage, fetal injury, or preterm labor.
Diagnostic tests include:
- Chorionic villus sampling: At 8 to 12 weeks, a tiny sample of placenta is obtained for analysis, using a needle inserted into the cervix or the abdomen.
- Amniocentesis: At 15 to 20 weeks, a small amount of amniotic fluid is obtained for analysis, using a needle inserted into the abdomen.
- Percutaneous umbilical blood sampling: After 20 weeks, a small sample of blood is taken from the umbilical cord for analysis, using a needle inserted into the abdomen.
Down syndrome can also be diagnosed after a baby is born by inspecting the infant's physical characteristics as well as blood and tissue.
There is no specific treatment for Down syndrome.
People with the syndrome will receive care for health problems, just as other people do. However, additional health screening for common problems may be recommended.
Early intervention can help the individual maximize their potential and prepare them to take an active role in the community.
Apart from physicians, special educators, speech therapists, occupational therapists, physical therapists, and social workers can all help. The National Institute for Child Health and Human Development (NICHHD) urge all participants to provide stimulation and encouragement.
Children who have specific difficulties with learning and development may be eligible for educational support, either in a mainstream or specialized school. In recent years, the tendency has increasingly been to attend mainstream schools, often with additional support to help them integrate and progress.
Some children will have an Individualized Education Plan (IEP), supported by various specialists.
The most common form of Down syndrome is known as trisomy 21, a condition where individuals have 47 chromosomes in each cell instead of 46.
Trisomy 21 is caused by an error in cell division called nondisjunction. This leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception. This variant accounts for 95 percent of Down syndrome cases.
The remaining 5 percent of Down syndrome cases are due to conditions called mosaicism and translocation.
Mosaic Down syndrome results when some cells in the body are normal while others have Trisomy 21.
Robertsonian translocation occurs when part of chromosome 21 breaks off during cell division and attaches to another chromosome, usually chromosome 14. The presence of this extra part of chromosome 21 causes some Down syndrome characteristics.
A person with a translocation does not have any special physical features, but they are more likely to have a child with an extra 21st chromosome.
A person with Down syndrome will be able to do many of the things that other people do. Children may take longer to acquire skills such as walking and talking, but with stimulation, they can acquire key life skills and attend school and, in some cases, college.
Depending on how the syndrome affects the individual, it is often possible for someone to work and to live semi-independently with Down syndrome.
Like anyone else, people with Down syndrome need friendships and relationships. Some will live with a partner or get married, and have an independent life. However, it is worth bearing in mind that if one partner has Down syndrome, there is a 30 to 50 percent chance that any children will too.
Average life expectancy is around 60 years, and many people with Down syndrome now live into their 70s.