Down syndrome affects nearly every system in the body, from the heart and digestive tract to hearing, vision, thyroid function, and cognitive development. It is caused by an extra copy of chromosome 21, and that additional genetic material influences how the body grows and functions from before birth through adulthood. Most people associate the condition with intellectual disability, but its reach is far broader.
The Genetic Basis
In 90 to 95 percent of cases, every cell in the body carries three copies of chromosome 21 instead of the usual two. This is called trisomy 21. About 2 to 4 percent of people with Down syndrome have a translocation, where part of chromosome 21 attaches to another chromosome. Another 2 to 4 percent have mosaic Down syndrome, meaning only some of their cells carry the extra chromosome. People with mosaicism can have milder features, though there is wide variation.
Cognitive and Language Development
Intellectual disability is one of the most consistent effects. IQ scores among children with Down syndrome average around 50, compared to 100 in the general population, though the range spans from mild to severe. This translates to slower learning, but most children do acquire reading, writing, and daily living skills with support.
Speech and motor skills are typically delayed. Many children understand language better than they can produce it, creating a gap between what they want to say and what comes out. Speech therapy started early in life helps close this gap. Motor milestones like sitting, crawling, and walking also arrive later, partly because of low muscle tone, which is present from birth in nearly all children with the condition.
Heart Defects
About half of all babies born with Down syndrome have a congenital heart defect. The most common is a hole between the chambers of the heart that disrupts normal blood flow. Many of these defects require surgical repair in the first year of life. Thanks to advances in pediatric cardiac surgery, outcomes have improved dramatically and heart repair is one of the biggest reasons life expectancy has climbed so sharply over the past several decades.
Hearing and Vision
Approximately 75 percent of people with Down syndrome experience some degree of hearing loss. Most of it is conductive, meaning it stems from fluid buildup or chronic ear infections rather than nerve damage. About 70 percent deal with recurring middle ear infections during childhood, which can compound speech delays if hearing problems go undetected.
Vision is affected too. Crossed eyes (strabismus) occur in roughly 19 to 47 percent of individuals. Cataracts, either present at birth or developing later, affect about 15 percent. Routine eye exams starting in infancy catch these issues early, and corrective lenses or surgery can make a significant difference in daily functioning and learning.
Thyroid Function
The thyroid gland, which controls metabolism and plays a key role in brain development, is frequently underactive in people with Down syndrome. About 1 percent of newborns with the condition have congenital hypothyroidism, and between 25 and 32 percent of infants and young children develop a milder form of low thyroid function. Because an underactive thyroid can quietly worsen cognitive development if untreated, blood tests are recommended in the newborn period, at six months, at one year, and at two years. Many international guidelines call for continued testing every one to two years throughout life.
Digestive System
Around 10 percent of children with Down syndrome are born with structural problems in the digestive tract. These can include blockages in the small intestine or abnormalities of the esophagus or anus, most of which are identified and surgically corrected shortly after birth. Hirschsprung disease, a condition where nerve cells are missing from part of the large intestine and prevent normal bowel movements, occurs in about 2 percent of children with Down syndrome, a rate far higher than in the general population.
Celiac disease, an immune reaction to gluten that damages the lining of the small intestine, affects roughly 6 to 10 percent of people with the condition. Symptoms like bloating, poor weight gain, and chronic diarrhea can overlap with other digestive issues, so screening is an important part of ongoing care.
Growth and Physical Features
Children with Down syndrome grow more slowly and tend to be shorter than their peers. Specialized growth charts exist because standard charts don’t accurately reflect their growth patterns. Obesity becomes a concern in childhood and adolescence, partly because of lower muscle tone, a slower metabolism, and thyroid issues.
The physical features most people recognize include a flattened facial profile, eyes that slant upward at the outer corners, small ears, a short neck, and a tongue that may protrude slightly. Hands and feet tend to be small, and many people have a single crease running across the palm instead of the typical two. These features vary in prominence from person to person.
Alzheimer’s Disease Risk
One of the most significant long-term effects involves the brain. Chromosome 21 carries the gene for a protein that, when overproduced, forms the plaques associated with Alzheimer’s disease. The result is a dramatically elevated risk: roughly one in three people with Down syndrome develop dementia symptoms in their 50s, and closer to two in three are affected after age 60. Not everyone develops Alzheimer’s, but the risk is high enough that cognitive monitoring in midlife is standard practice.
Life Expectancy
The trajectory of life expectancy tells a striking story about how medical care has changed outcomes. In 1960, the average lifespan for a person with Down syndrome was about 10 years. By 2007, it had risen to around 47 years. That leap is largely the result of better heart surgery, antibiotics for recurring infections, thyroid management, and improved early intervention programs. Many adults with Down syndrome now live into their 60s, hold jobs, and live semi-independently, though the degree of support needed varies widely from person to person.
Immune System and Infections
People with Down syndrome have differences in immune function that make them more susceptible to respiratory infections, ear infections, and certain autoimmune conditions. The high rates of celiac disease and thyroid dysfunction are partly rooted in this immune tendency. Children are more likely to be hospitalized for respiratory illnesses like pneumonia and bronchiolitis, especially in the first few years of life. Staying current on vaccinations and treating infections promptly are practical ways to reduce complications.
Musculoskeletal Effects
Low muscle tone, called hypotonia, is present in virtually all newborns with Down syndrome. It affects feeding in infancy and delays the motor milestones that follow. Joint hypermobility, or excessive looseness in the joints, is also common and can affect stability, particularly in the neck. A small percentage of children have instability in the upper spine that requires monitoring, especially before participation in contact sports. Physical therapy beginning in infancy helps build strength, coordination, and balance over time.