Trisomy 18 is caused by the presence of an extra copy of chromosome 18 in a baby’s cells. Instead of the usual two copies, there are three, which disrupts normal development and leads to serious physical and intellectual differences. The condition occurs in roughly 1 in 2,500 pregnancies in the United States, though the live birth rate is lower, around 1 in 8,600, because many affected pregnancies end in miscarriage or stillbirth.
How the Extra Chromosome Gets There
The root cause is a cell division error called nondisjunction. When the body creates egg or sperm cells, it splits its 46 chromosomes into two sets of 23. Sometimes the two copies of chromosome 18 fail to separate properly, so one egg or sperm ends up with two copies instead of one. When that cell is fertilized, the resulting embryo has three copies of chromosome 18.
This error happens randomly. It is not caused by anything either parent did or didn’t do during pregnancy. Most cases of trisomy 18 are not inherited, and the vast majority occur in families with no history of chromosomal conditions.
Three Types of Trisomy 18
Not all trisomy 18 cases look the same at the genetic level. The type depends on how and when the chromosomal error occurred.
Full trisomy 18 is the most common form. Every cell in the body carries three copies of chromosome 18. This happens when the nondisjunction error occurs before fertilization, in either the egg or the sperm.
Mosaic trisomy 18 is rarer and generally less severe. In this form, only some of the body’s cells have the extra chromosome while others are normal. This happens in one of two ways: either the original embryo started with three copies and some cells corrected themselves, or a division error occurred after fertilization so that only a portion of cells were affected. Because fewer cells carry the extra chromosome, symptoms can range from very mild to nearly as severe as full trisomy 18, depending on which tissues are involved.
Partial trisomy 18 is the least common. Only part of chromosome 18 is duplicated. This can happen when a parent carries a balanced translocation, meaning a piece of chromosome 18 has swapped places with part of another chromosome. The parent is healthy because they still have the right amount of genetic material overall, but they can pass on an unbalanced version to their child. This is the one form of trisomy 18 that can run in families.
Why Maternal Age Matters
The single strongest risk factor for trisomy 18 is the age of the mother at conception. The nondisjunction error occurs more frequently in older eggs, which have spent more years suspended in an incomplete stage of cell division. The longer that pause lasts, the more likely the chromosome pairs are to separate incorrectly.
The numbers illustrate the relationship clearly. Research on pregnancies in women of advanced maternal age found that the rate of trisomy 18 was about 1.9 per 1,000 at age 35, rose to 5.1 per 1,000 at age 40, and jumped to 37 per 1,000 at age 45. For each additional year of maternal age, the odds increased by roughly 18%. That said, because younger women have far more pregnancies overall, a significant number of trisomy 18 cases still occur in mothers under 35.
How the Extra Genes Cause Problems
Chromosome 18 contains at least 263 known genes. When three copies are present instead of two, cells produce roughly 50% more of the proteins those genes encode. This imbalance, called a gene dosage effect, throws off the tightly coordinated process of embryonic development.
Research at the UT Health San Antonio Chromosome 18 Clinical Research Center has been mapping which of those 263 genes are “dosage sensitive,” meaning which ones actually cause noticeable effects when duplicated. As of 2025, about 99 genes on chromosome 18 still have an unknown role when duplicated, so scientists can’t yet trace every feature of the syndrome back to a specific gene. What is clear is that the combined overproduction of many proteins at once disrupts heart formation, brain development, bone growth, and organ structure, which is why trisomy 18 affects so many body systems simultaneously.
How Trisomy 18 Is Detected
Most cases are identified during pregnancy rather than at birth. Screening can begin as early as the first trimester through a combination of blood markers and ultrasound measurements. If initial screening suggests a higher risk, a cell-free DNA blood test (often called NIPT) can be performed. This test analyzes fragments of fetal DNA circulating in the mother’s blood and has a sensitivity of about 93% for trisomy 18, with near-perfect specificity, meaning false positives are extremely rare.
Because NIPT is a screening test and not a definitive diagnosis, a positive result is typically followed by amniocentesis or chorionic villus sampling, both of which analyze actual fetal cells and provide a conclusive answer. These diagnostic tests can also distinguish between full, mosaic, and partial trisomy 18, which matters for understanding prognosis.
Recurrence in Future Pregnancies
For parents who have had one pregnancy affected by full trisomy 18, the recurrence risk is low but slightly higher than the general population’s baseline risk. The increase is modest, generally estimated at about 1% above the age-related background risk for most couples. Genetic testing of the parents can help clarify the picture. If either parent carries a balanced translocation involving chromosome 18, the recurrence risk is substantially higher and depends on the specific rearrangement involved.
For the mosaic and partial forms, recurrence risk varies. A genetic counselor can review the family’s specific chromosomal findings and provide a more personalized estimate. In cases where neither parent has a translocation and the trisomy was a random nondisjunction event, the odds of it happening again in a future pregnancy remain quite small.