Trisomy is a genetic condition that results in an extra copy of a chromosome. A person with trisomy will have 47 chromosomes instead of 46. Trisomy pregnancies can result in a live birth but many end prematurely in miscarriage. You can’t prevent trisomy conditions because they occur spontaneously, and your risk of having a child with a trisomy condition increases after age 35.
Trisomy is a genetic condition where there is an extra copy of a chromosome. Chromosomes are structures within the nucleus of cells that carry DNA, which is a thread-like structure that makes you unique. Your body has 23 pairs of chromosomes, which equals 46 total chromosomes. You acquire half of your chromosomes from each of your parents. When your body has an extra copy of a chromosome (trisomy), your body has 47 total chromosomes. Trisomy can result in a full-term baby but most often causes a miscarriage during the first three months of pregnancy.
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The correct pronunciation of trisomy is “try-sow-me.” When a number follows trisomy, that designates which chromosome has a third copy. The pronunciation stays the same in both references, with the addition of the number that follows, for example, “trisomy 21” is “try-sow-me-twenty-one.”
Your healthcare provider will identify a trisomy disorder based on which chromosome has a third copy, resulting in a numbered diagnosis. Since each chromosome has a different role in your body’s blueprint, where the third copy attaches can affect what genetic condition your baby will have. The most common trisomy conditions include:
In your genetic code, the 23rd pair of chromosomes are your sex cells that determine gender. Designations for sex cells are XX for female or XY for male, instead of as a number. When your cells divide, your sex cells can copy abnormally, causing a trisomy. Types of sex cell trisomies include:
Trisomy can occur in any pregnancy. The risk is higher in those over 35 years old who become pregnant, but the majority of babies born with trisomy are to parents younger than 35 years of age because there are statistically more births in those under 35 years.
The most common type of trisomy is trisomy 21 (Down syndrome). There are nearly 6,000 babies born with trisomy 21 each year in the United States, which equals about 1 in 700 babies.
During a prenatal ultrasound, your healthcare provider will look for signs of trisomy conditions including:
Depending on the type of trisomy diagnosed, your baby will have symptoms including:
Your chromosomes form in a specific order in your body. This sequence of cells is the blueprint of who you are. When cells form in reproductive organs, they start from one fertilized cell (sperm in males, eggs in females), then go through cell division (meiosis), where one cell splits twice to create four cells. Each cell contains half the amount of DNA as the original, so the copy contains 23 chromosomes from the original 46. Those four cells split evenly nine times to create more cells until you’ve replicated 23 pairs of cells. Numbers one through 23 designate each chromosome pair.
During meiosis (cell division), there is a chance that your cells do not divide as they should, causing an additional copy of a cell to join a pair. Normally, each pair would contain two chromosomes, but a third chromosome forms and attaches to the pair, creating a trisomy, which literally translates to “three bodies.”
Trisomy occurs during fertilization. It happens randomly and the diagnosis is not a result of something the parent did during pregnancy. People who become pregnant after age 35 are at a higher risk of having a child diagnosed with trisomy.
Genetic testing helps lead to a trisomy diagnosis early during pregnancy. Confirmation of the diagnosis occurs shortly after the baby is born, following a physical examination and further genetic chromosome testing of the infant by a blood sample.
Your healthcare provider will offer screening tests during pregnancy to test a sample of the birthing parent’s blood in addition to an ultrasound. During the ultrasound, your healthcare provider will look for signs of trisomy, like excess amniotic fluid, nuchal lucency (fluid under the skin behind the neck) and limb lengths. These can be signs of a genetic abnormality.
Following a screening, additional tests help confirm the diagnosis including:
Trisomy is a lifelong condition and requires long-term treatment to alleviate any symptoms associated with the condition. Treatment for children born with trisomy includes:
You can’t prevent genetic conditions like trisomy from occurring because the chromosome error occurs randomly during cell division. You can reduce your risk of having a child with a genetic condition by:
An extra chromosome changes the blueprint for your baby and will cause physical abnormalities that are present at birth (congenital defects), like distinct facial features along with intellectual disabilities. Many children born with trisomy will have subsequent health concerns that arise following their diagnosis like frequent ear infections, heart problems or sleep apnea, but with adequate treatment, your child will be able to live a happy and full life.
The survival rate for babies born with trisomy 18 or trisomy 13 is low after the first few weeks of life (neonatal period) due to the severity of the condition, especially delayed or abnormal organ development. Your healthcare provider will assess your baby’s health and offer treatment to increase the survival rate for children born with these conditions.
One of the side effects of trisomy is risk of miscarriage. This would normally occur during the first three months of pregnancy. Contact your healthcare provider if you experience any symptoms of a miscarriage including:
Trisomy disorders occur when you have an extra copy of a chromosome. Monosomy occurs when you are missing a copy of a chromosome. Both of these genetic conditions are the result of a genetic mutation where your cells don’t divide as they should. You can’t prevent this abnormality from happening during cell division.
A note from Cleveland Clinic
Since there is no way to prevent genetic abnormalities like trisomy conditions, if you plan on becoming pregnant, talk with your healthcare provider about genetic testing to assess your risk of having a child with a genetic condition. If you become pregnant and your baby receives a trisomy diagnosis, there is support available to help you and your baby live healthy and fulfilling lives. Genetic counseling offers resources to help you understand your baby’s diagnosis so you can provide them with the care and support they need as they grow.
Last reviewed by a Cleveland Clinic medical professional on 05/03/2022.
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