Babies get Down syndrome when they have an extra copy of chromosome 21, giving them three copies instead of the usual two. This extra genetic material alters development in ways that affect both physical features and learning. Roughly 1 in every 1,000 to 1,100 babies worldwide is born with the condition, and in most cases it happens randomly during the earliest stages of reproduction, not because of anything either parent did or didn’t do.
What Happens at the Chromosome Level
Human cells normally contain 23 pairs of chromosomes, for a total of 46. When an egg or sperm cell is being made, each pair has to split apart so the resulting cell carries just one copy of each chromosome. Down syndrome occurs when this splitting process fails for chromosome 21. The technical name for that failure is nondisjunction, and it accounts for about 95% of all Down syndrome cases.
When an egg or sperm with two copies of chromosome 21 joins with a normal partner at fertilization, the embryo ends up with three copies in every single cell. Because every cell traces back to that one fertilized egg, the extra chromosome is present body-wide. This is called complete trisomy 21, and it’s the most common form of Down syndrome by a wide margin.
Less Common Forms
Mosaic Down Syndrome
In fewer than 5% of cases, the chromosome error doesn’t happen before fertilization. Instead, a normal egg and sperm unite, but something goes wrong in one of the very first cell divisions afterward. The result is a mix: some cells have the standard 46 chromosomes while others carry the extra chromosome 21. This is called mosaic Down syndrome, and because not every cell is affected, the traits associated with the condition can be milder, though that varies widely from person to person. In some instances, mosaic trisomy 21 occurs when an embryo that started with three copies loses the extra chromosome in certain cell lines early in development.
Translocation Down Syndrome
In a small number of cases, the total chromosome count looks normal, but part or all of an extra chromosome 21 is physically attached to another chromosome, often chromosome 14. This is called a translocation. Sometimes it happens spontaneously. Other times, a parent carries a “balanced” version of the translocation, meaning they have the right amount of genetic material overall but it’s rearranged. That parent is healthy, but they have a higher chance of passing on the extra chromosome 21 material to a child. A balanced carrier of a Robertsonian translocation involving chromosome 21 has roughly a 25% theoretical risk of having a baby with Down syndrome in any given pregnancy. This is the only form of Down syndrome that can genuinely run in families.
Why Maternal Age Matters
The single biggest known risk factor is the age of the mother at conception. The reason comes down to biology: a woman’s eggs begin forming before she is born and remain suspended in an incomplete state of cell division for decades. The longer those eggs sit in that paused state, the more likely the chromosome-separation process is to go wrong when the egg finally matures.
The numbers tell the story clearly. At age 25, the chance of having a baby with Down syndrome is about 1 in 1,250. By 31, it rises to 1 in 1,000. At 35, the risk is roughly 1 in 400. By 40, it reaches about 1 in 100, and by 45, estimates put it at around 1 in 30. That steep curve after 35 is why age-related screening is so commonly discussed during pregnancy.
The Father’s Age Plays a Role Too
For a long time, research focused almost entirely on the mother’s age. But a large Columbia University study examining over 3,400 Down syndrome cases found that the father’s age matters more than previously thought, especially when the mother is also older. Men over 40 were twice as likely to have a child with Down syndrome compared to men under 20. When both parents were over 40, the rate of Down syndrome was roughly 60 per 10,000 births, a six-fold increase compared to couples where both parents were under 35.
In the group where both parents were 40 or older, the paternal contribution to the extra chromosome was estimated at 50%. The researchers suggested that because older women tend to have children with older men, some of the risk traditionally attributed to maternal age alone is likely the combined effect of both parents’ ages. Advanced paternal age appears to increase the frequency of chromosome abnormalities in sperm, adding to the overall risk.
Can It Be Inherited?
The vast majority of Down syndrome cases are not inherited. Complete trisomy 21 and mosaic trisomy 21 are essentially random events. They are not caused by a gene passed down through generations, and having one child with either form doesn’t mean a parent carries any genetic trait for the condition.
Translocation Down syndrome is the exception. If a parent is a balanced translocation carrier, they can pass on the rearranged chromosome in multiple pregnancies. Genetic testing can identify carriers, and this is one of the key reasons families with a child who has translocation Down syndrome are often referred for genetic counseling. Knowing the type of Down syndrome matters for understanding whether future pregnancies carry an elevated risk.
How It’s Detected During Pregnancy
Prenatal testing falls into two categories: screening tests that estimate risk, and diagnostic tests that give a definitive answer.
Screening options include a blood test called cell-free DNA screening (often referred to as NIPT), which can be done in any trimester. It analyzes fragments of the baby’s DNA circulating in the pregnant person’s bloodstream and flags a higher or lower likelihood of conditions like Down syndrome. Another common option is the quad screen, a second-trimester blood test measuring four substances that, together, help estimate risk. Neither screening test can confirm Down syndrome on its own. False positives and false negatives do occur, and the rates vary by test.
If a screening test suggests elevated risk, or if age, family history, or other factors warrant it, diagnostic tests provide a definitive answer. Chorionic villus sampling (typically done in the first trimester) and amniocentesis (usually in the second trimester) both collect a small sample of cells with the baby’s genetic material. These are the only tests that can confirm whether the baby has Down syndrome. Both carry a small risk of miscarriage, which is part of why they’re generally offered after a screening test rather than as a first step.
Risk in Future Pregnancies
Parents who already have a child with Down syndrome often want to know what their chances are in a subsequent pregnancy. For complete trisomy 21, the recurrence risk is slightly higher than the baseline for the mother’s age, but still relatively low. The increase is thought to reflect the possibility that some parents have a biological predisposition to nondisjunction, though researchers haven’t pinpointed exactly why this happens in most cases.
For translocation Down syndrome, the recurrence risk depends on whether a parent is a carrier and which chromosomes are involved. Genetic counseling can map out these specifics using a blood test called a karyotype, which produces a detailed picture of a person’s chromosomes. This is typically one of the first steps recommended after a baby is diagnosed with translocation Down syndrome.