Patau syndrome is caused by an extra copy of chromosome 13 in a baby’s cells, a condition known as trisomy 13. It occurs in roughly 1 in 10,000 births and is one of the most serious chromosomal conditions, with about 87% of affected infants not surviving the first year of life. The extra genetic material disrupts normal development, leading to severe abnormalities of the brain, heart, and other organs.
How the Extra Chromosome Happens
Every cell in your body normally contains 23 pairs of chromosomes, for a total of 46. Patau syndrome occurs when a baby ends up with three copies of chromosome 13 instead of two. The most common way this happens is through a cell division error called nondisjunction.
During the formation of egg or sperm cells, the paired chromosomes are supposed to separate evenly. When they fail to split properly, one reproductive cell ends up with an extra copy of chromosome 13 while the other gets none. If the cell carrying the extra chromosome is the one involved in fertilization, every cell in the resulting embryo will contain three copies of chromosome 13. This accounts for the vast majority of Patau syndrome cases, and it happens randomly. Parents don’t carry any genetic flaw that causes it.
Translocation and Mosaic Forms
Not all cases follow the same pattern. About 1 in 10 cases result from a chromosomal translocation, where a piece of chromosome 13 breaks off and attaches to another chromosome, most commonly chromosome 14. The total amount of chromosome 13 material is still excessive, which produces the same effects. The key difference is that translocation can sometimes be inherited. One parent may carry a balanced version of the rearrangement (with no symptoms themselves) and pass an unbalanced version to the child. This is the one scenario where Patau syndrome can run in a family, and genetic testing of the parents can clarify the risk.
A rarer form, called mosaic trisomy 13, occurs when the cell division error happens after fertilization rather than before it. Because the error strikes later, only some of the baby’s cells carry the extra chromosome while others are normal. The severity of symptoms depends largely on what percentage of cells are affected and which tissues they’re in. Some individuals with mosaic trisomy 13 have relatively mild features, while others experience the full range of complications. This variability makes the mosaic form harder to predict.
Why Maternal Age Matters
The risk of trisomy 13 rises significantly with maternal age. At age 20, the rate is approximately 1 per 10,000 pregnancies. By age 40, that number jumps to about 14 per 10,000. The underlying reason is biological: as eggs age over the decades they spend in the ovaries, the structures that hold chromosome pairs together gradually weaken. The protein “glue” binding chromosomes deteriorates, and the cellular machinery that checks for errors during division becomes less reliable. These age-related changes make it more likely that chromosomes will fail to separate correctly.
One study using non-invasive prenatal screening found that no women under 39 with normal egg reserve tested positive for trisomy 13, 18, or 21. Only women aged 39 and older with diminished egg reserve had positive results. While younger women can still have an affected pregnancy, the probability is substantially lower.
How the Extra Chromosome Affects Development
Chromosome 13 contains instructions for building and regulating many different tissues. When cells have three copies instead of two, they produce too much of certain proteins, throwing off the tightly coordinated process of fetal development. The effects are wide-ranging and severe.
In a study of 240 live-born infants with trisomy 13, the most common abnormalities were:
- Heart defects: 57% of infants, including holes between heart chambers and malformed valves
- Orofacial clefts: 45%, meaning cleft lip, cleft palate, or both
- Extra fingers or toes (polydactyly): 44%
- Nervous system abnormalities: 39%, most notably a brain malformation called holoprosencephaly, where the brain fails to divide into two hemispheres properly
- Eye abnormalities: 30%, including unusually small or underdeveloped eyes
Holoprosencephaly is particularly significant because it disrupts both brain function and the development of midface structures. When the brain doesn’t divide normally, the face often follows, which is why many infants with Patau syndrome have closely set eyes or a single central eye, a small or absent nose, and a small head overall. The neurological impairment from this malformation is severe and is a major factor in the condition’s poor prognosis.
Survival and Prognosis
Patau syndrome carries one of the highest mortality rates of any chromosomal condition. According to data from the CDC spanning multiple registries, roughly half of live-born infants with trisomy 13 do not survive the first week of life. Within the first year, mortality reaches 87%. About 10% of infants do survive past their first birthday, though most of these children have profound developmental disabilities and ongoing medical needs.
Children with the mosaic form tend to have better outcomes than those with full trisomy 13, particularly when a lower percentage of their cells carry the extra chromosome. However, even in mosaic cases, the range of outcomes is extremely variable and difficult to predict before birth.
Prenatal Detection
Trisomy 13 can be detected during pregnancy through several methods. Non-invasive prenatal testing (NIPT), a blood test performed as early as 10 weeks of pregnancy, analyzes fragments of fetal DNA circulating in the mother’s blood. A large study of nearly 69,000 pregnancies found that NIPT detected trisomy 13 with 100% sensitivity and 99.95% specificity, meaning it catches virtually all true cases with very few false alarms.
NIPT is a screening test, not a diagnostic one. A positive result is typically followed by amniocentesis or chorionic villus sampling, which analyze fetal cells directly and can confirm the diagnosis with certainty. These tests can also determine whether the trisomy is full, mosaic, or translocation-based, which matters for understanding severity and recurrence risk.
Recurrence Risk for Future Pregnancies
For parents who have had a child with Patau syndrome caused by nondisjunction, the risk of it happening again in a future pregnancy is about 1%. That is higher than the general population risk, but still low. The slight increase may reflect a predisposition to cell division errors that isn’t fully understood.
When the cause is a translocation, the picture changes. If one parent carries a balanced translocation, the chance of passing along an unbalanced version is significantly higher and depends on which chromosomes are involved and which parent carries the rearrangement. Genetic counseling and parental chromosome testing (called karyotyping) can clarify the specific risk. For this reason, all families affected by Patau syndrome are typically offered genetic testing to determine the underlying mechanism and guide planning for future pregnancies.