Down syndrome is caused by the presence of an extra copy of chromosome 21, giving a person 47 chromosomes instead of the usual 46. This extra genetic material disrupts normal development, leading to the characteristic physical features and intellectual differences associated with the condition. The error almost always originates during the formation of an egg or sperm cell, when chromosomes fail to separate properly during cell division.
How the Extra Chromosome Happens
Every egg and sperm cell is supposed to carry exactly one copy of each chromosome. During the cell division process that creates these reproductive cells, paired chromosomes must pull apart and sort into separate cells. When chromosome 21 fails to separate correctly, one cell ends up with two copies and the other with none. If the cell with two copies becomes part of a fertilized egg, the resulting embryo has three copies of chromosome 21 instead of two. This failure to separate is called nondisjunction.
At least 90% of these errors occur in the egg, not the sperm. Among those maternal errors, the majority happen during the first stage of cell division. The process relies on physical connections between paired chromosomes that hold them together and help them line up properly on the cell’s internal scaffolding. When those connections form in the wrong spot, or don’t form at all, the chromosomes are at risk of sorting unevenly. A connection positioned too close to the tip or too close to the center of chromosome 21 increases the likelihood of a separation error.
Three Types of Down Syndrome
Not every case of Down syndrome looks the same at the genetic level. There are three distinct forms, and they differ in how the extra chromosome 21 material ends up in a person’s cells.
Trisomy 21
This is the most common form, accounting for about 95% of cases. Every cell in the body carries a full extra copy of chromosome 21. It results from the nondisjunction error described above and is not inherited from a parent’s genes.
Translocation
In roughly 5% of cases, a person has the usual two copies of chromosome 21, but an additional piece of chromosome 21 is physically attached to a different chromosome (often chromosome 14). The total amount of chromosome 21 material is still increased, which produces the same effects. Some translocation cases arise spontaneously, but others are inherited from a parent who carries a “balanced” rearrangement. A balanced carrier has all the right amount of genetic material, just rearranged, so they show no symptoms themselves. However, they can pass the unbalanced version to a child. A carrier of a typical balanced translocation involving chromosome 21 has roughly a 25% theoretical chance of having a child with Down syndrome in each pregnancy. In the rare situation where a parent carries a translocation with both copies of chromosome 21 fused together, all viable pregnancies will result in Down syndrome.
Mosaic
Fewer than 5% of people with Down syndrome have the mosaic form. Here, the initial fertilized egg has the normal 46 chromosomes, but a cell division error occurs shortly after conception. Some cells end up with an extra chromosome 21 while others remain typical. The result is a mix: some cells have 47 chromosomes and some have 46. People with mosaic Down syndrome sometimes have milder features, depending on how many cells carry the extra chromosome and where in the body those cells are concentrated.
Why Maternal Age Matters
A mother’s age at conception is the strongest known risk factor. The probability rises sharply after 35. At age 25, the chance of having a baby with Down syndrome is about 1 in 1,250. By 31, it’s roughly 1 in 1,000. At 35, it jumps to about 1 in 400, and by 40, it reaches approximately 1 in 100.
The reason ties back to egg biology. Women are born with all the eggs they will ever have, and those eggs sit suspended in the middle of cell division for decades. The structures that hold chromosome pairs together degrade over time. By the time an egg completes its division at ovulation, those weakened connections may not do their job, allowing chromosomes to sort incorrectly. This age-related deterioration is why the majority of nondisjunction errors are maternal.
That said, the majority of babies with Down syndrome are born to mothers under 35, simply because younger women have far more pregnancies overall.
The Paternal Age Factor
The father’s age plays a smaller but real role, particularly when both parents are older. A study published in The Journal of Urology found no significant parental age influence on Down syndrome risk until both parents reached 35 or older. Among mothers 40 and older, the father’s age accounted for roughly 50% of the increased risk. When both parents were over 40, the rate of Down syndrome was six times higher than in couples where both were under 35. Fathers 40 and older paired with mothers 35 and older showed a twofold increase compared to fathers under 25 in the same maternal age group.
These findings suggest that age-related changes in sperm, while less dominant than those in eggs, can compound the risk when combined with advanced maternal age.
What the Extra Chromosome Does
Chromosome 21 is the smallest human chromosome, but it still contains hundreds of genes. Having three copies instead of two means the body produces roughly 50% more of the proteins those genes code for. This overproduction disrupts finely tuned developmental processes, particularly in the brain and heart.
Researchers have identified a small stretch of chromosome 21, located in a region called 21q22.2, that appears especially important. Studies of people with only a partial extra copy of chromosome 21 have narrowed down a 1.6 million base-pair segment that seems to drive many of the condition’s hallmark features. Several genes within this region are actively expressed during early development, and their overactivity is thought to contribute to the cognitive and physical characteristics of Down syndrome. The full picture is more complex, though, as genes outside this critical region also play a role.
Is Down Syndrome Inherited?
In the vast majority of cases, no. Trisomy 21 and mosaic Down syndrome are random events. They are not caused by anything a parent did or didn’t do, and having one child with either form does not substantially raise the risk for future pregnancies beyond what age-related factors would predict.
The exception is translocation Down syndrome. About one-third of translocation cases are inherited from a carrier parent. If a child is diagnosed with translocation Down syndrome, genetic testing of both parents can determine whether one of them carries a balanced translocation. If so, genetic counseling can help the family understand recurrence risks for future pregnancies. The other two-thirds of translocation cases arise spontaneously, with no family history.
How Down Syndrome Is Detected During Pregnancy
Prenatal detection involves two tiers: screening tests that estimate probability, and diagnostic tests that provide a definitive answer.
The most common screening approach today is a blood test that analyzes fragments of fetal DNA circulating in the mother’s bloodstream. This test is highly sensitive but not perfect. In a large Dutch study of 237 pregnancies flagged as having Down syndrome, 227 were confirmed, while 10 were false positives. Six babies with Down syndrome were missed entirely. The test’s reliability also depends heavily on the mother’s baseline risk. For a woman in her 20s with no other risk factors, a positive result is correct only about 46% of the time. For someone who already has an elevated risk of 1 in 150, that accuracy rises to around 87%.
Diagnostic confirmation requires amniocentesis, which analyzes cells from the fluid surrounding the baby. Another option, chorionic villus sampling, takes cells from the placenta, but because it samples placental tissue rather than fetal tissue, it can occasionally give misleading results due to differences between placental and fetal cells. For this reason, amniocentesis is generally considered the more reliable confirmation test, especially after a positive screening result.