What is Trisomy 13?

By design, humans are each supposed to have 23 pairs of chromosomes for a total of 46. But sometimes, errors can occur. When an individual has three copies of a particular chromosome instead of two in all the cells of their body, they are said to have a trisomy.

Trisomy 13, also known as Patau’s syndrome, is a genetic disorder that occurs when there are three copies of chromosome 13, instead of the standard two copies. In practical terms, this means that a fetus develops certain abnormalities because they have too many copies of a particular chromosome. 

There are a few types of trisomy 13 that can occur:

• Full trisomy 13, which occurs when an individual has three complete copies of chromosome 13
• Partial trisomy 13, which occurs when a person has two full copies of chromosome 13, plus an additional part of a third copy
• Mosaic trisomy 13, which occurs when only some of the body’s cells have three copies of chromosome 13

Trisomy 13 is considered rare, with approximately 1 in every 16,000 newborns affected. Ahead, find out more about this chromosomal abnormality, how it occurs, and what it means for a baby with this condition. 

Causes of Trisomy 13

Experts don’t fully know why a fetus would have three copies of chromosome 13. Most cases seem to occur randomly during the formation of the sperm and eggs in the parents.  

However, there is one possible way for an individual to inherit a trisomy 13. A person who has a condition called translocation trisomy 13—meaning, their 13th chromosome and another chromosome have become rearranged—could potentially pass down extra material from their 13th chromosome to an offspring. 

While anyone could theoretically have a baby with trisomy 13, the risk increases as you age.

Signs of Trisomy 13

According to the Genetic and Rare Diseases Information Center, severe intellectual disability is one of the hallmarks of trisomy 13. However, it can cause other physical traits and abnormalities such as:

• Brain and spinal cord abnormalities
• Congenital heart defects
• Microphthalmia, or abnormally small eyes
• Polydactyly, or extra toes or fingers
• Cleft lip
• Cleft palate
• Hypotonia, or weak muscle tone

How is Trisomy 13 Diagnosed?

Your doctor may recommend starting with an ultrasound. A first-trimester ultrasound can pick up early possible indicators, such as an increased thickness of the tissue at the back of fetus’ neck. It can also reveal an abnormally high fetal heart rate or significant abnormalities, like an usually large bladder.

“Ultrasound technology has improved to the point that it is now the initial method of screening fetuses for physical malformations consistent with trisomy 13,” says Alex Juusela, MD, an OB/GYN at Detroit Medical Center/Wayne State University. 

Dr. Juusela says he recommends that his pregnant patients undergo a maternal blood test, also known as non-invasive prenatal testing or screening, that looks for fragments of DNA from the placenta in the maternal blood. However, the currently available cell-free DNA tests were designed to detect trisomy 21 and are less accurate in the detection of trisomy 13.

“This is why first and second trimester ultrasound screening should be used in combination with either or both of the aforementioned maternal blood screening tests,” he says. 

That said, it’s important to understand that screenings are not perfect. That’s why a diagnostic test such as amniocentesis (testing of the amniotic fluid) or chorionic villus sampling (of the placenta tissue) is necessary, says Dr. Juusela. These genetic tests can provide more definitive information for a diagnosis.

Prognosis for Babies with Trisomy 13

When a baby is born with trisomy 13, they may require surgical intervention, according to Angela Bianco, MD, director of maternal fetal medicine at the Mount Sinai Health System and professor of obstetrics, gynecology, and reproductive science at the Icahn School of Medicine at Mount Sinai. For example, surgical repair for congenital heart disease, which is common among trisomy 13 babies, may be an option to help them live a little longer.

Unfortunately, most babies who are born with trisomy 13 do not live very long. As Dr. Bianco puts it, “The survivability over one year for trisomy 13 infants is very low.”

Approximately one in 10 babies born with Patau’s syndrome will live for more than one year, and it’s nearly always the ones diagnosed with a partial or mosaic form.

Coping with a Trisomy 13 Diagnosis

Learning that your unborn baby has trisomy 13 can be a shock. Parents may need to first discuss their reactions with each other, then weigh their options with their physician as they decide how to proceed. 

“Parents will also benefit from having a shared decision model for the mode of delivery," says Dr. Bianco. "Some women may choose not to undergo a cesarean delivery, given low survival rates for trisomy 13 newborns."

Although it may not help you in the moment, it might comfort you down the road to learn that you likely won’t have another pregnancy affected by trisomy 13 if you try to conceive again. “There is an approximately 1% risk of recurrence,” explains Dr. Bianco.

A Word From Verywell

Learning that your unborn child may be affected by a chromosomal disorder can be shocking, especially since trisomy 13 is a rare condition. You may feel frightened, sad, or even angry. At the very least, you may feel stunned—and have many questions. It’s okay to need a little time to decide how you feel and what you want to do. Talk to your OB/GYN or healthcare provider about your options so you can make the best choice for you and your family.