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.


A child diagnosed with trisomy could have facial characteristics that include slanted eyes, a round face or a cleft palate.
The genetic condition trisomy means that your child has an extra chromosome, which could cause your child to be born with unique facial characteristics.

What is trisomy?

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.


Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

How do you pronounce trisomy?

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.”

What are the different types of trisomy disorders?

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:

  • Trisomy 13 (Patau syndrome).
  • Trisomy 18 (Edward syndrome).
  • Trisomy 21 (Down syndrome).

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 X (XXX).
  • Klinefelter syndrome (XXY).
  • Jacob’s syndrome (XYY).


Who does trisomy affect?

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.

How common is trisomy?

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.


Symptoms and Causes

What are the symptoms of trisomy during pregnancy?

During a prenatal ultrasound, your healthcare provider will look for signs of trisomy conditions including:

  • Excess amniotic fluid surrounding your baby.
  • Only one artery in your umbilical cord.
  • Small placenta.
  • Your baby is less active.
  • Your baby is small for its age.
  • Physical abnormalities like heart problems or a cleft palate.

What are the symptoms of trisomy after my baby is born?

Depending on the type of trisomy diagnosed, your baby will have symptoms including:

  • Small stature.
  • Round face with a flat profile.
  • Slanted eyes.
  • Cleft palate.
  • Organs forming or working abnormally (heart, lungs or kidney).
  • Developmental delays and intellectual disabilities.

What causes trisomy?

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.

Diagnosis and Tests

How is trisomy diagnosed?

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.

What tests diagnose trisomy conditions?

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:

  • Chorionic villus sampling (CVS): Between 10 and 13 weeks of pregnancy, your healthcare provider will take a small sample of cells from your placenta to test for genetic conditions and the sex of your baby.
  • Amniocentesis: Between 15 and 20 weeks of pregnancy, your healthcare provider will take a small sample of amniotic fluid to test for potential health abnormalities in your baby.
  • Percutaneous umbilical blood sampling (PUBS): Your healthcare provider will take a small sample of blood from your baby’s umbilical cord to test for health conditions in your baby.
  • Noninvasive prenatal testing: After 10 weeks of pregnancy, your healthcare provider will examine a sample of your blood to assess whether or not your child has any genetic abnormalities.

Management and Treatment

How are trisomy conditions treated?

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:

  • Surgery to repair any physical abnormalities.
  • Educational support.
  • Speech, behavioral and physical therapy.
  • Medicines to help symptoms of related medical conditions that develop over time.


How can I reduce my risk of having a child with trisomy?

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:

  • Understanding your risk of becoming pregnant if you are over 35 years old.
  • Getting genetic screenings done before becoming pregnant.
  • Avoiding using tobacco products and drinking alcohol.
  • Taking care of your health by eating a well-balanced diet and exercising regularly.

Outlook / Prognosis

How will trisomy affect my baby?

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.

Living With

When should I see my healthcare provider?

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:

  • Cramping.
  • Pain in your lower back.
  • Abdominal pain.
  • Light to heavy bleeding.
  • Chills or a fever.

What questions should I ask my doctor?

  • How do I reduce my risk of having a child with a genetic disorder like trisomy?
  • What prenatal screenings do you recommend to verify if my baby has a genetic condition?
  • Can I have a successful pregnancy after a trisomy diagnosis?

Additional Common Questions

What is the difference between trisomy and monosomy?

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.

Medically Reviewed

Last reviewed on 05/03/2022.

Learn more about our editorial process.

Questions 216.444.2538