Down syndrome is a genetic condition caused by an extra copy of chromosome 21. It affects about 1 in every 640 babies born in the United States, with roughly 5,700 new cases each year. The extra chromosome changes how the body and brain develop, leading to a recognizable set of physical features, some degree of intellectual disability, and a higher risk for certain medical conditions.
How the Extra Chromosome Happens
Human cells normally contain 46 chromosomes, arranged in 23 pairs. Down syndrome occurs when a person ends up with three copies of chromosome 21 instead of two. This error, called nondisjunction, happens during the formation of an egg or sperm cell. Normally, chromosome pairs split evenly so each new cell gets one copy. In nondisjunction, both chromosomes from a pair end up in the same cell, leaving the other cell with none. When that egg or sperm joins with a normal one at conception, the resulting embryo has 47 chromosomes.
This is largely a random event. It can happen in any pregnancy, though the likelihood increases with maternal age. In the vast majority of cases (around 95%), every cell in the body carries the extra chromosome. This is called complete trisomy 21.
Three Types of Down Syndrome
Not all cases look the same at the genetic level. There are three forms:
- Trisomy 21 is the most common, accounting for about 95% of cases. Every cell has three full copies of chromosome 21.
- Translocation makes up about 3–4% of cases. Here, part of chromosome 21 breaks off and attaches to another chromosome. The total amount of genetic material from chromosome 21 is still extra, but it’s arranged differently. This is the only type that can sometimes be inherited from a parent who carries the translocation without showing symptoms.
- Mosaic Down syndrome accounts for only 1–2% of cases. Some cells have the extra chromosome and others don’t. People with mosaicism may have fewer or milder features, though this varies widely.
Physical Features
Babies with Down syndrome often share certain physical traits that doctors can spot at birth. These include upward-slanting eyes, often with a small skin fold covering the inner corner of the eye, and a single deep crease running across the palm of the hand instead of the typical two. Poor muscle tone is one of the earliest and most consistent signs. Newborns may feel noticeably floppy when held.
Other common features include a flattened facial profile, a smaller head, a short neck, and small ears. None of these features cause health problems on their own, and they vary from person to person. Some children have many of these traits, others only a few.
Developmental Milestones
Children with Down syndrome reach the same developmental milestones as other children, but on a slower timeline. On average, babies gain head control around 6 months, sit independently by 12 months, and walk independently around 28 months. For comparison, most typically developing children walk between 9 and 15 months. Speech and vocabulary development are also delayed, with language often being a particular area of challenge that benefits from early speech therapy.
Intellectual disability ranges from mild to moderate in most cases. The degree varies significantly from one person to another. Many children with Down syndrome attend mainstream schools with support, and adults can hold jobs, maintain friendships, and live with varying degrees of independence. Early intervention programs that begin in infancy, including physical therapy, occupational therapy, and speech therapy, make a measurable difference in long-term outcomes.
Heart Defects and Other Medical Conditions
Between 50 and 65% of babies born with Down syndrome also have a congenital heart defect. Some of these are mild and resolve on their own as the child grows. Others require surgery, sometimes in the first year of life. Advances in pediatric heart surgery have been one of the biggest factors in improving survival and quality of life over the past several decades.
Thyroid dysfunction is another common issue, particularly hypothyroidism, which slows metabolism and can contribute to weight gain and fatigue if untreated. Routine blood tests can catch it early, and it’s easily managed.
Sleep apnea is strikingly common, affecting 40–88% of adults with Down syndrome and up to 97% of children. By comparison, only 7–13% of the general population deals with sleep apnea. The combination of lower muscle tone, a smaller airway, and a larger tongue all contribute. Untreated sleep apnea disrupts sleep quality and can affect mood, behavior, and learning, so screening is an important part of ongoing care.
Alzheimer’s Risk in Adulthood
Chromosome 21 carries a gene involved in producing a protein called beta-amyloid, which plays a central role in Alzheimer’s disease. Having an extra copy of this gene means the brain produces more of this protein over a lifetime. By age 40, most people with Down syndrome have significant buildup of these protein deposits in the brain. An estimated 50% or more will eventually develop dementia due to Alzheimer’s disease, typically earlier than it appears in the general population. This connection has made the Down syndrome community an important focus for Alzheimer’s research more broadly.
Prenatal Screening and Diagnosis
Down syndrome can be detected before birth, but it’s important to understand the difference between screening tests and diagnostic tests. Screening tests estimate the probability that a baby has the condition. They don’t give a definitive answer.
Non-invasive prenatal testing (NIPT), a blood test that analyzes fragments of fetal DNA circulating in the mother’s blood, has detection rates as high as 99% for Down syndrome, with false-positive rates as low as 0.03–0.2% depending on the lab. Older screening methods, like first-trimester blood work combined with an ultrasound, detect about 75–80% of cases.
If a screening test comes back positive, confirmation requires a diagnostic test. The two options are amniocentesis (sampling fluid from around the baby, typically done at 15–20 weeks) and chorionic villus sampling (taking a tiny piece of tissue from the placenta, done at 10–13 weeks). These tests analyze the baby’s actual chromosomes and are considered definitive.
Life Expectancy and Quality of Life
Life expectancy for people with Down syndrome has increased dramatically. In 1960, the average was around 10 years, largely because heart defects went unrepaired and infections were harder to treat. Today, many people with Down syndrome live into their 60s, and some well beyond that. Better cardiac surgery, routine health monitoring, and early educational intervention have all driven this change.
Adults with Down syndrome increasingly live semi-independently or in supported housing, work in a range of jobs, and participate in community life. The degree of support needed varies, but the trajectory over the past few decades has moved steadily toward greater inclusion and higher expectations for what people with Down syndrome can achieve.