Trisomy means having three copies of a particular chromosome instead of the usual two. Human cells normally contain 46 chromosomes arranged in 23 pairs, so a trisomy results in 47 total chromosomes. The extra chromosome disrupts normal development and can cause a wide range of physical and intellectual differences depending on which chromosome is affected.
How Trisomy Happens
Trisomy originates from a cell division error called nondisjunction. When egg or sperm cells form, paired chromosomes are supposed to separate so each resulting cell gets one copy. In nondisjunction, that separation fails, and both copies get pulled to the same side. One resulting cell ends up with an extra chromosome, and the other ends up short one.
If an egg or sperm carrying that extra chromosome is involved in fertilization, the embryo will have three copies of that chromosome in every cell. This error can happen at two different stages of cell division. When it occurs at the earlier stage, all four resulting egg or sperm cells are abnormal. When it happens at the later stage, two of the four cells are normal and two are not.
Three Types of Trisomy
Not every trisomy looks the same at the genetic level. About 95% of trisomy cases are “full” trisomy, meaning every cell in the body carries the extra chromosome. This happens when the error occurs before fertilization, during egg or sperm formation.
In mosaic trisomy, the error happens after fertilization, during one of the early cell divisions of the embryo. The result is a mix: some cells have the normal 46 chromosomes, and others have 47. Mosaicism accounts for roughly 2% of cases (using Down syndrome as the most-studied example) and often produces milder features, since a portion of cells function normally.
Translocation trisomy, also about 2% of cases, is different again. The total chromosome count may appear normal, but extra material from one chromosome is physically attached to another. This type is sometimes inherited from a parent who carries the rearrangement without symptoms, which matters for understanding the chance of it happening again in future pregnancies.
The Most Common Trisomies
Most trisomies are incompatible with life and result in early miscarriage. Only a few are survivable, and each has distinct characteristics.
Trisomy 21 (Down Syndrome)
The most common and most survivable trisomy involves an extra copy of chromosome 21. It causes intellectual disability that ranges from mild to moderate, along with characteristic facial features, low muscle tone in infancy, and an increased risk of heart defects. Many people with Down syndrome live into their 60s and beyond, and with early support, most achieve meaningful independence.
Trisomy 18 (Edwards Syndrome)
An extra chromosome 18 causes severe developmental problems affecting nearly every organ system. Babies are typically thin and frail, with a small head and jaw, low-set ears, clenched fists, and contracted joints. About 90% have heart defects. Spina bifida, seizures, kidney problems, and scoliosis can also occur. Trisomy 18 is far more serious than trisomy 21, and most affected pregnancies end in miscarriage or stillbirth. Among those born alive, survival beyond the first year is uncommon.
Trisomy 13 (Patau Syndrome)
An extra chromosome 13 causes the most severe of the survivable trisomies. The brain often fails to divide properly during development, a condition called holoprosencephaly. Babies may have a sloping forehead, close-set eyes, underdeveloped nostrils, extra fingers and toes, and heart and kidney defects. Some are born with abdominal organs protruding through an opening near the umbilical cord. Like trisomy 18, most affected pregnancies do not survive to term, and life expectancy for those born alive is very limited.
Sex Chromosome Trisomies
Trisomy can also involve the sex chromosomes (X and Y), and these conditions are generally much milder than autosomal trisomies. Many people with sex chromosome trisomies are never diagnosed because their symptoms are subtle.
Klinefelter syndrome (47,XXY) is the most common, affecting about 1 in 650 male newborns. The extra X chromosome leads to smaller testes that produce less testosterone. Without treatment, this can mean delayed puberty, breast tissue growth, reduced muscle mass, lower bone density, and less facial and body hair. Most people with Klinefelter syndrome are 2 to 3 inches taller than expected for their family. Fertility is often affected. Children may experience mild delays in speech and language, an increased risk of learning disabilities (particularly with reading), and higher rates of ADHD and anxiety.
Triple X syndrome (47,XXX) affects females and often causes no noticeable physical differences. Some individuals are taller than average and may have mild learning difficulties. XYY syndrome, where males carry an extra Y chromosome, is similarly subtle, with above-average height being the most consistent feature.
Why Maternal Age Matters
The risk of trisomy rises sharply with the age of the egg-producing parent. A large Danish study of more than 500,000 pregnancies quantified this clearly, using women aged 20 to 29 as the baseline. Women aged 35 to 39 had roughly 4.7 times the risk of any chromosomal abnormality. At 40 to 44, the risk jumped to about 16 times higher. At 45 and older, it was 36 times higher.
This increase happens because eggs are formed before birth and sit in a paused state of cell division for decades. The longer they wait, the more likely the chromosome-separation machinery is to malfunction. Sperm, by contrast, are produced continuously, so paternal age plays a much smaller role in trisomy risk.
How Trisomy Is Detected
Prenatal screening for trisomy has become highly accurate. Noninvasive prenatal testing (NIPT), a blood test that analyzes fragments of fetal DNA circulating in the pregnant person’s blood, can detect trisomy 21 with about 99% sensitivity, and trisomy 18 and 13 at 98% to 99% sensitivity, with a combined false-positive rate of just 0.13%. It also performs well for sex chromosome trisomies, with sensitivity and specificity near or above 99% for most types.
NIPT is a screening test, though, not a diagnostic one. A positive result means the probability is high, but confirmation requires amniocentesis or chorionic villus sampling. These procedures analyze fetal cells directly and are considered the gold standard for definitive diagnosis. They carry a small risk of complications, which is why they’re typically offered after a positive screening result rather than as a first step.
After birth, trisomy is confirmed through a blood test called a karyotype, which produces a visual map of all 46 (or 47) chromosomes. This test also reveals whether the trisomy is full, mosaic, or translocation, which is important for understanding the likely severity and for counseling families about the chance of recurrence in future pregnancies.