What Is a Trisomy?

These Chromosomal Abnormalities Sometimes Carry a Miscarriage Risk

Chromosomes, human karyotype, down's syndrome, trisomy 21
Chromosomes, human karyotype, down's syndrome, trisomy 21. Getty Images/SMC Images

A trisomy is a chromosome abnormality in which there are three copies of a particular chromosome rather than the normal two. Certain trisomies can cause miscarriages

What Are Chromosomes and Trisomies?

Chromosomes are structures located inside every cell that contain a person's genes. Trisomies are a type of aneuploidy, a condition in which there are missing or extra chromosomes. In a trisomy, there is an extra copy - or third - chromosome.

Normally, each cell has 23 pairs of chromosomes: chromosomes 1 - 22, and the sex chromosomes (X and Y). Trisomies are named for the chromosome pair that has the extra copy.

Doctors believe these chromosome abnormalities are caused by random errors in cell division.

Trisomies are more common in older mothers, although it is not understood why advanced maternal age increases the risk.

How Trisomies Affect Health

Trisomies can have varying effects on a person's health, depending on which chromosome is affected by the trisomy. 

Some trisomies, such as trisomy X, have only mild effects on a person, whereas others such as trisomy 21 (also known as Down syndrome) usually cause health problems and developmental delays. Certain trisomies, such as trisomy 13, trisomy 16 and trisomy 18, are associated with miscarriages or loss of the baby shortly after birth. At least 50 percent of miscarriages are believed to be caused by chromosomal anomalies including trisomies.


All trisomies are relatively rare, but these are the most common ones and their effects on the health of the person who has them:

Trisomy X - Triple X syndrome. About 1 in 1,000 females have this condition, in which there are three X chromosomes instead of two. Some people with trisomy X may have no symptoms or mild symptoms, while others may have more serious manifestations of the syndrome, including developmental delays and learning disabilities.

The most common physical feature of triple X syndrome is taller than average height with long legs. 

Trisomy 21 - Down syndrome.This is the most common genetic condition in the U.S., affecting one out of every 800 infants. People with down syndrome have abnormal features of the face and body, and they may have health problems like heart defects, intellectual disability and dementia.

Trisomy 13 - Patau syndrome and Trisomy 18 - Edwards syndrome. Trisomy 13 and 18 are rarer than trisomy 21. Babies with trisomy 13 or 18 have birth defects that cause intellectual disabilities and affect nearly every organ of the body. Sadly, most infants with trisomy 13 and 18 die within the first year of life. 

Trisomy 16. This is the most common trisomy associated with miscarriage. According to the DOC16 Foundation, it's estimated that trisomy 16 causes 100,00 miscarriages every year. Fetuses do not survive full trisomy 16, although some children are born with trisomy 16-related disorders.

Prenatal Tests for Trisomies

There are two types of tests you can undergo during pregnancy to look for trisomies.


  • Screening tests. These tests (blood tests, ultrasound) identify the possibility that your baby will have different chromosome problems and neural tube defects. They cannot not tell you if your fetus actually has a trisomy.
  • Diagnostic tests. These tests (usually amniocentesis or chorionic villus sampling) tell you if your fetus has a chromosomal abnormality or an inherited disorder.

It's up to you to decide if you want to undergo amniocentesis or chorionic villus sampling to diagnose trisomies and other chromosomal abnormalities or inherited disorders. 

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