Down syndrome is a genetic condition caused by an extra copy of chromosome 21. Instead of the usual 46 chromosomes, a person with Down syndrome has 47. This extra genetic material changes how the body and brain develop, leading to a recognizable set of physical features and some degree of intellectual disability. It is one of the most common chromosomal conditions, occurring in roughly 1 in every 700 births in the United States.
What Causes the Extra Chromosome
Human cells normally contain 23 pairs of chromosomes, half from each parent. Down syndrome happens when chromosome 21 doesn’t separate properly during the formation of an egg or sperm cell, a process called nondisjunction. The result is a reproductive cell with two copies of chromosome 21 instead of one. When that cell joins with a normal cell from the other parent, the embryo ends up with three copies of chromosome 21.
This error in cell division is more likely to occur as a woman ages. At age 25, the chance of having a baby with Down syndrome is about 1 in 1,300. By 35, that rises to 1 in 365. At 45, it jumps to 1 in 30. Maternal age is the strongest known risk factor, though Down syndrome can occur at any age. Research has also linked changes in how chromosome 21 pairs swap genetic material during cell division. When those exchanges happen less frequently or in unusual positions along the chromosome, it raises the odds that the chromosomes won’t separate correctly.
Three Genetic Types
Not every case of Down syndrome looks the same at the chromosomal level. There are three distinct types:
- Trisomy 21 accounts for 90 to 95 percent of cases. Every cell in the body carries a full extra copy of chromosome 21.
- Translocation makes up 2 to 4 percent of cases. Part or all of the extra chromosome 21 is attached to a different chromosome. This is the only type that can sometimes be inherited from a parent who carries a rearranged chromosome without showing symptoms.
- Mosaic Down syndrome also accounts for 2 to 4 percent of cases. Only some cells have the extra chromosome while others have the typical 46. People with mosaicism may have fewer or milder features, depending on how many cells are affected and which tissues contain them.
Physical and Developmental Features
Certain physical features are usually noticeable at birth and become more apparent as a child grows. These commonly include a flattened facial profile (especially across the bridge of the nose), almond-shaped eyes that slant upward, a short neck, small ears and hands, and a single crease running across the palm. Many babies also have low muscle tone, which makes them feel floppy when held, and they tend to be shorter than average throughout life.
Cognitive development varies widely. Most people with Down syndrome have mild to moderate intellectual disability, which means they learn more slowly and may need more time to reach milestones like sitting, walking, and talking. Children often understand more than they can express verbally, and speech development in particular tends to lag behind other skills. That said, many children attend mainstream schools with support, and adults with Down syndrome hold jobs, maintain friendships, and live with varying degrees of independence.
Common Health Conditions
The extra chromosome affects more than appearance and learning. About half of all babies born with Down syndrome have a congenital heart defect, making a cardiac evaluation one of the first things doctors check after birth. Some defects are minor and resolve on their own; others require surgery in the first year of life. Advances in pediatric heart surgery are one of the main reasons survival rates have improved so dramatically.
Beyond the heart, people with Down syndrome face higher rates of several other conditions throughout life. These include thyroid problems (especially an underactive thyroid), hearing loss, vision issues, sleep apnea, and gastrointestinal problems. Low muscle tone and joint looseness can also affect posture and movement, though physical activity and therapy help significantly.
Alzheimer’s Risk in Adulthood
Chromosome 21 carries a gene involved in producing a protein that, when it accumulates in the brain, forms the plaques associated with Alzheimer’s disease. Because people with Down syndrome have three copies of this gene, their brains produce more of this protein from a young age. By age 40, most people with Down syndrome have detectable brain changes associated with Alzheimer’s. An estimated 50 percent or more will eventually develop dementia symptoms as they age, typically earlier than in the general population.
This doesn’t mean every person with Down syndrome will develop Alzheimer’s, and having brain plaques doesn’t automatically cause symptoms. But it does mean that cognitive changes in middle age should be taken seriously and evaluated rather than attributed to the condition itself.
Prenatal Screening and Diagnosis
There is an important distinction between screening tests and diagnostic tests during pregnancy. Screening tests estimate the probability of Down syndrome but cannot confirm it. Diagnostic tests can.
Screening options include a first-trimester combined test (a blood draw plus an ultrasound measuring fluid at the back of the baby’s neck) and an integrated test that combines first- and second-trimester blood work. Cell-free DNA testing, available from 10 weeks of pregnancy, analyzes fragments of fetal DNA circulating in the mother’s blood and is highly accurate, though a positive result still needs confirmation.
Diagnostic tests provide a definitive answer. Chorionic villus sampling (CVS) takes a small sample of placental tissue between 10 and 14 weeks. Amniocentesis draws a sample of the fluid surrounding the baby, typically after 15 weeks. Both involve directly examining the baby’s chromosomes. These tests carry a small risk of miscarriage, so they are generally offered when screening suggests elevated risk or when parents want certainty.
Early Intervention and Therapies
Early intervention programs can begin shortly after birth and often continue until age 3. Research consistently shows that starting therapy early improves developmental outcomes. A child with Down syndrome will typically work with a team that includes physical therapists, speech therapists, occupational therapists, and special educators.
Physical therapy focuses on building muscle strength, improving balance, and helping children reach motor milestones like crawling and walking. Speech therapy addresses both language comprehension and expression, often incorporating sign language or picture boards while verbal skills catch up. Occupational therapy helps with fine motor skills in early childhood, things like grasping objects and self-feeding, and later shifts toward practical life skills. By high school, occupational therapists often help teenagers identify career interests and develop workplace abilities.
These therapies aren’t a short-term fix. Many people with Down syndrome continue to benefit from targeted support throughout their school years and into adulthood, adapting the focus as their needs and goals change.
Life Expectancy Today
The outlook for people with Down syndrome has changed enormously. In 1960, the average life expectancy was around 10 years, largely because heart defects and infections went untreated. By 2007, that number had risen to about 47 years. Improved heart surgery, better treatment of infections, and broader access to healthcare all contributed to this shift. Many individuals today live into their 50s and 60s.
Longer lives have brought new considerations, including the Alzheimer’s risk mentioned earlier and a greater focus on adult healthcare, housing, and employment. Communities increasingly recognize that support for people with Down syndrome doesn’t end at childhood, and programs for independent or assisted living, job coaching, and social engagement continue to expand.