Trisomy 13 and trisomy 18 are chromosomal conditions in which a baby has three copies of a particular chromosome instead of the usual two. Trisomy 13 (also called Patau syndrome) involves an extra copy of chromosome 13, while trisomy 18 (Edwards syndrome) involves an extra copy of chromosome 18. Both cause serious physical and developmental problems, and most affected pregnancies end in miscarriage or stillbirth. Among live births, trisomy 18 is roughly twice as common as trisomy 13, occurring in about 1 in 9,300 births compared to about 1 in 18,000.
How the Extra Chromosome Happens
Both conditions usually result from a random error during the formation of eggs or sperm. When reproductive cells divide, the chromosome pairs are supposed to separate evenly. Sometimes they don’t, a mistake called nondisjunction. If an egg or sperm ends up with an extra copy of chromosome 13 or 18, and that cell is involved in conception, the resulting baby will carry three copies in every cell. In trisomy 13, the error occurs in the mother’s egg about 91% of the time, typically during the earliest stage of cell division.
This full form, with three complete copies in every cell, accounts for roughly 80% of trisomy 13 cases and the large majority of trisomy 18 cases. Two less common forms also exist:
- Translocation: The extra chromosome material is attached to another chromosome rather than existing as a free-standing third copy. In trisomy 13, this usually involves chromosomes 13 and 14 fusing together. Unlike the full form, translocation can sometimes be inherited from a parent who carries a balanced rearrangement without symptoms.
- Mosaic: Only some of the body’s cells have the extra chromosome while the rest are normal. About 5% of trisomy 13 cases are mosaic. Children with mosaic forms tend to be less severely affected, though the range is wide depending on how many cells carry the extra copy.
In the vast majority of families, neither parent did anything to cause the condition, and most cases are not inherited. The recurrence risk for future pregnancies is approximately 1%.
Maternal Age and Risk
Advanced maternal age is a well-established risk factor for trisomy 18. The rate climbs steeply: roughly 1.9 per 1,000 pregnancies at age 35, 5.1 per 1,000 at age 40, and 37 per 1,000 at age 45. For each additional year of maternal age, the odds increase by about 18%. Interestingly, research from large population studies has not found the same strong age correlation for trisomy 13, suggesting that the nondisjunction errors behind trisomy 13 may be driven by somewhat different biological factors.
Physical Features of Trisomy 13
Trisomy 13 tends to affect brain and facial development most severely. The brain may fail to divide properly into two hemispheres, a condition called holoprosencephaly, which in turn drives many of the facial features. Babies often have cleft lip and palate, closely set eyes, and a small head. Extra fingers or toes (polydactyly) are a hallmark finding, along with a single crease across the palm instead of the typical two.
Heart defects occur in up to 80% of affected infants. Holes between the heart chambers are common, as are more complex structural problems. Kidney abnormalities are also frequent. The overall picture is one of multiple organ systems being affected simultaneously, which is what makes the condition so serious.
Physical Features of Trisomy 18
Trisomy 18 has its own distinct pattern. Babies are typically very small at birth due to restricted growth in the womb. The most recognizable feature is a characteristic hand position: clenched fists with the index fingers overlapping the other fingers. Feet may have a rounded or “rocker-bottom” shape. The jaw is often unusually small, and the mouth opening is narrow.
Heart defects are equally common in trisomy 18, again affecting up to 80% of infants. The most frequent is a hole between the lower chambers of the heart (ventricular septal defect), found in nearly half of affected children. More complex heart malformations also occur. Other findings can include an opening in the abdominal wall, a small head with a flattened back, and abnormalities of the kidneys.
Prenatal Screening and Diagnosis
Most families first learn about a possible trisomy through routine prenatal screening. Non-invasive prenatal testing (NIPT), which analyzes fragments of fetal DNA circulating in the mother’s blood, can be done as early as 10 weeks of pregnancy. For trisomy 18, NIPT detects about 96% of cases. For trisomy 13, the detection rate is slightly lower in large pooled studies, around 91%, though individual studies have reported higher figures. False positive rates for both are very low, around 0.13%.
A positive screening result is not a diagnosis. Confirmation requires a diagnostic test that examines the baby’s actual chromosomes. Chorionic villus sampling (CVS) can be performed between 10 and 12 weeks of pregnancy, while amniocentesis is typically done between 15 and 18 weeks. Both involve collecting a small sample of fetal cells and provide a definitive answer. Some families first receive a suspicion of trisomy based on ultrasound findings, such as growth restriction, heart defects, or clenched hands, which can prompt diagnostic testing.
Survival and Prognosis
The prognosis for both conditions is serious, but the range of outcomes is wider than many families are initially told. Median survival after birth is about 5 days for trisomy 13 and 8 days for trisomy 18. Many infants die within the first week from heart failure, breathing difficulties, or other complications of their organ malformations.
However, a meaningful number of children do survive longer. A multi-state population study covering births from 1999 to 2007 found that 11.5% of children with trisomy 13 and 13.4% of children with trisomy 18 were alive at one year. At five years, survival was 9.7% for trisomy 13 and 12.3% for trisomy 18. Children with mosaic forms or less severe organ involvement tend to have better outcomes.
These numbers represent a shift in how the medical community talks about these conditions. For decades, trisomy 13 and 18 were described as “incompatible with life,” and comfort care was often the only option offered. More recent evidence has challenged that framing. While early mortality remains high, the fact that some children survive months or years has pushed guidelines toward individualized decision-making rather than a uniform palliative approach.
How Care Decisions Are Made
There is no single treatment for trisomy 13 or 18, because the extra chromosome affects virtually every organ system. Instead, care is tailored to the specific problems each child has. For some families, the focus is on comfort and quality of life from the start. For others, particularly when the heart defect is the primary life-threatening issue, surgical intervention may be discussed.
Heart surgery for children with trisomy 13 or 18 was once considered inappropriate, but a growing number of centers now evaluate these children on a case-by-case basis. The decision depends on the severity of the heart defect, the burden of other malformations, and the family’s goals. Current recommendations emphasize multidisciplinary teams working with families to develop a care plan that reflects the individual child’s condition rather than applying a blanket policy based on the chromosomal diagnosis alone.
Early integration of palliative care specialists, even when active treatment is pursued, has become standard practice at many centers. Palliative care in this context does not mean giving up. It means ensuring that pain is managed, that families are supported, and that care decisions are revisited as the child’s condition evolves.