Down syndrome is caused by the presence of an extra copy of chromosome 21, giving a baby 47 chromosomes instead of the usual 46. This happens because of a cell division error that typically occurs before or very shortly after conception, not because of anything a parent did during pregnancy. Globally, it affects roughly 1 in 1,000 to 1 in 1,100 live births.
How the Extra Chromosome Happens
Every egg and sperm cell is supposed to contain exactly one copy of each chromosome. To get there, cells go through a special type of division called meiosis, where chromosome pairs split apart. Sometimes the pair of chromosome 21 doesn’t separate properly. This error, called nondisjunction, means one egg or sperm ends up with two copies of chromosome 21 instead of one. When that cell joins with a normal egg or sperm at fertilization, the resulting embryo has three copies of chromosome 21.
About 95% of the time, this error comes from the mother’s egg cell rather than the father’s sperm. Among those maternal cases, roughly 77% of the errors happen during the first stage of egg cell division, and about 23% during the second stage. In the small number of cases traced to the father’s sperm, the pattern flips: most errors occur during the second stage of division.
Three Types of Down Syndrome
Not every case of Down syndrome looks the same at the genetic level. There are three distinct forms, and the type matters for understanding recurrence risk in future pregnancies.
Full trisomy 21 accounts for 90 to 95% of all cases. Every cell in the baby’s body carries three copies of chromosome 21. This is almost always a random event during egg or sperm formation and is not inherited from the parents.
Translocation Down syndrome makes up about 1 to 5% of cases. Here, an extra piece of chromosome 21 is physically attached to another chromosome (often chromosome 14). This can happen randomly, but it can also be inherited from a parent who carries a “balanced” translocation, meaning they have all the right genetic material, just rearranged, and show no symptoms themselves. A balanced carrier parent theoretically has up to a 25% chance of having a child with Down syndrome, though the actual observed risk varies. In the rare situation where a parent carries a translocation involving both copies of chromosome 21, all viable pregnancies will result in Down syndrome.
Mosaic Down syndrome is the least common form, occurring in roughly 1 to 4% of cases. Only some of the baby’s cells have three copies of chromosome 21 while others have the typical two. This happens when the cell division error occurs after fertilization, during early embryo development. People with mosaic Down syndrome sometimes have milder features, though this varies widely.
Why Maternal Age Is the Biggest Risk Factor
A woman’s age at the time of conception is the strongest known predictor of having a baby with Down syndrome. A 20-year-old has approximately a 1 in 1,980 chance. By age 36, that risk climbs to about 1 in 267. The increase accelerates sharply after 35, which is why prenatal screening is routinely offered to women in that age group.
The reason is biological. Women are born with all the eggs they will ever have. These eggs remain paused mid-division for decades. The longer an egg sits in that suspended state, the more likely the chromosome pairs are to separate incorrectly when division finally resumes at ovulation. This accumulation of risk over time explains why nondisjunction errors are far more common in eggs than in sperm, which are produced fresh on an ongoing basis.
Does the Father’s Age Matter?
Paternal age plays a smaller but real role. Older men produce more sperm with chromosome errors. Research from Bosnia and Herzegovina found that the highest percentage of children with Down syndrome were born to fathers aged 30 and older, with a notable share (about 32%) born to fathers over 40. The effect of paternal age is harder to isolate because older fathers tend to have older partners, but studies increasingly recognize it as an independent contributing factor.
Recurrence Risk in a Future Pregnancy
If you’ve had a previous pregnancy with Down syndrome caused by full trisomy 21, the recurrence risk is about 1%, regardless of your age. That’s higher than the general population risk for most women, likely because a small percentage of parents carry the chromosome error in some of their egg or sperm cells (called gonadal mosaicism) even though their own blood tests show normal chromosomes.
Translocation cases are different. If genetic testing reveals that one parent carries a balanced translocation, the recurrence risk can be substantially higher and depends on the specific chromosomes involved. Genetic counseling can clarify the exact risk for your situation. If the translocation arose randomly (de novo) and neither parent is a carrier, the recurrence risk is minimal.
Prenatal Screening and Diagnosis
Screening tests estimate the likelihood that a pregnancy is affected, while diagnostic tests give a definitive answer. Both are available, and they differ in timing, accuracy, and what they involve.
Screening With Cell-Free DNA (NIPT)
Non-invasive prenatal testing analyzes fragments of the baby’s DNA circulating in the mother’s blood. It can be done as early as 10 weeks of pregnancy. For Down syndrome specifically, NIPT detects over 97% of cases across all available methods, with specificity above 99%. That means false positives are rare, but they do happen. A positive NIPT result is not a diagnosis on its own. It needs confirmation through a diagnostic procedure.
Diagnostic Testing
Two procedures can confirm Down syndrome with certainty by directly analyzing the baby’s chromosomes. Chorionic villus sampling (CVS) can be performed after 10 weeks of pregnancy, making it the earlier option. It involves taking a small tissue sample from the placenta. Amniocentesis is typically performed around 16 weeks and involves collecting a sample of amniotic fluid. Both carry a small risk of miscarriage, which is why they’re generally offered after a screening test suggests elevated risk rather than as a first step.
Results from amniocentesis are usually available after 17 weeks of gestation. CVS results come sooner because the procedure is done earlier, giving parents more time to prepare or make decisions.
What Doesn’t Cause Down Syndrome
Down syndrome is not caused by anything a parent ate, drank, or was exposed to during pregnancy. It is not related to stress, physical activity, medications, or lifestyle choices. The chromosome error either occurs during egg or sperm formation before conception or, in the case of mosaic Down syndrome, in the very earliest cell divisions after fertilization. Environmental factors have not been shown to trigger nondisjunction in human studies. The two established risk factors are parental age and, for translocation cases, carrier status in a parent.