Trisomy 13, also called Patau syndrome, is a genetic condition in which a baby has three copies of chromosome 13 instead of the usual two. This extra chromosome disrupts normal development and causes serious birth defects affecting the brain, heart, and face. It is one of the most severe chromosomal conditions, and most affected infants do not survive beyond their first year of life.
What Causes Trisomy 13
Every cell in the body normally contains 23 pairs of chromosomes, for a total of 46. Trisomy 13 occurs when something goes wrong during cell division, most often when the egg or sperm is forming. Instead of each new cell getting one copy of chromosome 13, a cell ends up with two. After fertilization, the resulting embryo carries three copies in every cell. This error, called nondisjunction, happens randomly and typically originates on the mother’s side.
There are three forms of the condition, and the type matters for both severity and recurrence risk:
- Full trisomy 13 accounts for about 80% of cases. Every cell in the body carries the extra chromosome. It results from a random error in cell division and is not inherited.
- Translocation trisomy 13 occurs when extra material from chromosome 13 becomes attached to another chromosome, most often chromosome 14. This form can be sporadic, but in some families one parent carries a balanced rearrangement of their own chromosomes without any symptoms. That parent can pass along the unbalanced form to a child, which means the condition can recur in future pregnancies.
- Mosaic trisomy 13 makes up roughly 5% of cases. The error happens after fertilization, so only some cells carry the extra chromosome while others are normal. Because normal cells are mixed in, the physical effects tend to be fewer and milder.
How It Affects the Body
The extra chromosome 13 disrupts development across nearly every organ system. The most recognizable effects involve the brain and face. The brain often fails to divide into two distinct hemispheres, a condition called holoprosencephaly. Because facial development is closely linked to how the brain forms, this leads to midline facial differences: cleft lip and palate, very small or absent eyes, a sloping forehead, small jaw, and unusually shaped ears.
Heart defects are present in the majority of infants. The most common are holes between the chambers of the heart, though more complex structural problems also occur. These heart defects are a leading cause of early death.
Other features include extra fingers or toes (usually on the pinky side of the hand), curved feet, and restricted growth before birth. The kidneys, liver, digestive tract, and reproductive organs can also be affected. More than half of affected babies have abnormalities in the reproductive and urinary systems, while conditions like abdominal wall defects and abnormal kidney structure occur in a smaller proportion.
How Trisomy 13 Is Detected
Most cases are identified during pregnancy through a combination of screening and diagnostic testing. A common first step is noninvasive prenatal testing (NIPT), a blood draw from the mother that analyzes fragments of fetal DNA circulating in her bloodstream. In a large study of nearly 69,000 pregnancies, NIPT detected 100% of trisomy 13 cases that were later confirmed. While highly accurate, NIPT is still a screening test, meaning a positive result needs confirmation.
Confirmation comes through amniocentesis or chorionic villus sampling (CVS), both of which collect actual fetal cells so the chromosomes can be examined directly. These are the gold standard for diagnosing any chromosomal condition. Ultrasound findings, such as heart defects, brain abnormalities, or extra fingers, often prompt testing in the first place.
Prognosis and Survival
Full trisomy 13 carries a very poor prognosis. The median survival is measured in days to weeks, and the majority of infants with the full form do not reach their first birthday. The severity of heart and brain defects plays the largest role in determining how long a baby survives.
Mosaic trisomy 13 follows a notably different path. Because only a fraction of cells carry the extra chromosome, children with this form often present with fewer and less severe anomalies. Some survive into late childhood or adulthood, and there is documented improvement in motor and cognitive abilities over time. Skin pigment changes, both lighter and darker patches, appear in about 82% of people with the mosaic form and can sometimes be the feature that prompts genetic testing in an older child.
Medical Care and Treatment Options
The approach to caring for a baby with trisomy 13 has shifted meaningfully in recent years. Historically, comfort-focused care was the only option offered to families. That is still a valid and respected choice, but it is no longer the only one. The American Academy of Pediatrics now emphasizes shared decision making, recognizing that a trisomy 13 diagnosis alone should not automatically rule out medical interventions that could prolong life or allow a baby to go home from the hospital.
One increasingly common approach is a “trial of therapy,” where doctors provide short-term support for breathing difficulties and other treatable problems in the newborn period. This gives families time with their baby while clinicians assess how the infant responds. If the baby stabilizes, further decisions can be made based on how things unfold rather than on the diagnosis alone.
Heart surgery is now performed in some cases. Multiple studies show it can improve outcomes, though surgical risks remain higher than for babies without a chromosomal condition. In 2024, the American Association for Thoracic Surgery published guidance stating that decisions about heart surgery should be made case by case, weighing the severity of the heart defect alongside other anomalies, rather than being ruled out simply because of the trisomy. Other surgeries for conditions like bowel obstructions or abdominal wall defects may also be considered when they align with a family’s goals. Families are counseled that surgical interventions in this population carry longer hospital stays and increased risks from anesthesia.
Recurrence Risk for Future Pregnancies
For families affected by trisomy 13, one of the most pressing questions is whether it could happen again. When the cause is the common form, a random nondisjunction error, the recurrence risk is low. It was a one-time event during the formation of a single egg or sperm cell.
The translocation form is different. If one parent carries a balanced chromosomal rearrangement, they have an elevated risk of having another child with trisomy 13 in each subsequent pregnancy. Genetic testing of both parents after a diagnosis can determine whether a translocation was inherited or occurred spontaneously. When a parent does carry a balanced translocation, genetic counseling helps map out the specific risks and available options, including preimplantation genetic testing during IVF, for future pregnancies.