Patau syndrome, also called Trisomy 13, occurs in roughly 1 in 10,000 to 1 in 16,000 live births, making it one of the rarest of the three major chromosomal trisomies. It is significantly less common than Down syndrome (Trisomy 21), which occurs in about 1 in 700 births, and somewhat less common than Edwards syndrome (Trisomy 18). However, these live birth figures only tell part of the story, since many pregnancies affected by Trisomy 13 end in miscarriage before reaching full term.
What Causes Patau Syndrome
Patau syndrome happens when a baby has three copies of chromosome 13 instead of the usual two. In most cases, this is “full trisomy,” meaning every cell in the body carries the extra chromosome. The error occurs randomly during the formation of eggs or sperm, when chromosomes fail to separate properly during cell division.
About 20% of cases involve a translocation, where part of chromosome 13 breaks off and attaches to another chromosome. This distinction matters because translocation forms can sometimes be inherited from a parent who carries the rearranged chromosomes without being affected themselves. A small number of cases involve mosaicism, where only some cells have the extra chromosome. Mosaic forms tend to produce milder symptoms.
Maternal Age and Risk
Like other chromosomal trisomies, the risk of Patau syndrome rises with maternal age. The underlying mechanism is the same one behind the age-related increase in Down syndrome: as eggs age over a woman’s lifetime, they become more prone to errors in chromosome separation. One study from Thailand found that the combined rate of the three major trisomies (13, 18, and 21) ranged from about 4.5 per 1,000 pregnancies in younger women to nearly 100 per 1,000 in the oldest age groups, though Trisomy 13 accounts for the smallest share of those numbers.
Still, Patau syndrome can occur at any maternal age. The majority of babies with Trisomy 13 are born to mothers under 35 simply because younger women have far more pregnancies overall.
How It’s Detected During Pregnancy
Most cases of Patau syndrome are now identified before birth through a combination of ultrasound and blood-based screening. At the 11- to 14-week ultrasound, several markers can raise suspicion. In a study of 181 affected pregnancies, 78% had increased fluid at the back of the neck (nuchal translucency above the 95th percentile), 71% had an unusually fast fetal heart rate, and 50% had visible structural abnormalities like brain malformation, abdominal wall defects, or an enlarged bladder. Combining these markers allows detection of more than 90% of affected pregnancies during the first trimester.
Non-invasive prenatal testing (NIPT), which analyzes fragments of fetal DNA circulating in the mother’s blood, can also screen for Trisomy 13. NIPT picks up virtually all cases, with 100% sensitivity in one large study spanning 2018 to 2021. However, its positive predictive value for Trisomy 13 is only about 18%, meaning most positive results turn out to be false alarms. This is partly because Trisomy 13 is so rare that even a very accurate test will flag more false positives than true cases. A positive NIPT result is always followed by confirmatory testing, typically amniocentesis or chorionic villus sampling, before a diagnosis is considered definitive.
Physical Effects on the Baby
Patau syndrome affects nearly every organ system. Among the most common findings in live-born infants: heart defects occur in about 57% of cases, extra fingers or toes (polydactyly) in 44%, cleft lip or palate in 45%, nervous system abnormalities in 39%, and eye abnormalities in 30%. Many babies also have brain development problems, including holoprosencephaly, a condition where the brain fails to divide into two hemispheres properly.
Other features include a small head, closely spaced or fused eyes, clenched fists with overlapping fingers, and rocker-bottom feet. The combination of severe heart defects and brain malformations is what makes the condition so life-threatening.
Survival and Long-Term Outlook
Patau syndrome carries one of the most serious prognoses of any chromosomal condition. According to a 2025 clinical report from the American Academy of Pediatrics, only 43% of live-born infants with Trisomy 13 survive beyond the first week of life. Between 10% and 25% survive past their first birthday, with the range depending on the study and the level of medical intervention provided. About 9.7% survive beyond five years.
For the small percentage of children who make it through infancy, longer survival is possible. Some individuals have lived into their twenties, though this typically involves ongoing medical and technological support. Children with mosaic Trisomy 13, where only some cells carry the extra chromosome, or partial trisomy from a translocation generally have better survival odds and may have less severe symptoms, though significant developmental delays are still expected.
Recurrence Risk for Future Pregnancies
For parents who have had one pregnancy affected by Trisomy 13, the chance of recurrence depends on the underlying cause. When the condition resulted from a random error in chromosome separation (the most common scenario), the recurrence risk is low, generally estimated at about 1% above the baseline age-related risk.
When the cause is a Robertsonian translocation carried by one parent, the situation is different. These parents carry a rearranged chromosome that increases the odds of producing an egg or sperm with an extra copy of chromosome 13. Even so, the empirical risk of having a live-born baby with translocation Trisomy 13 remains under 0.4%, because most affected pregnancies miscarry early. Genetic counseling and carrier testing can clarify the specific risk for individual families, and preimplantation genetic testing is an option for those pursuing IVF.