What Is Trisomy 14? Causes, Symptoms, and Outlook

Trisomy 14 is a chromosomal condition in which a person has three copies of chromosome 14 instead of the usual two. In its complete form, it is almost always fatal before birth, with the vast majority of affected pregnancies ending in early miscarriage. The cases that do result in a live birth nearly always involve mosaicism, meaning only some of the body’s cells carry the extra chromosome while others are normal. Even in its mosaic form, trisomy 14 is extraordinarily rare, with only about 30 cases documented in the medical literature as of recent reports.

Why Most Cases Never Reach Birth

Having a full extra copy of chromosome 14 in every cell is incompatible with sustained development. The vast majority of pregnancies with complete trisomy 14 end in spontaneous abortion during the first trimester. This is why the condition is far more commonly seen in miscarriage tissue samples than in live-born children. When clinicians study products of conception after early pregnancy loss, trisomy 14 turns up as one of several autosomal trisomies that the developing embryo simply cannot survive.

The rare survivors are those with mosaic trisomy 14, where a mixture of normal cells (46 chromosomes) and trisomic cells (47 chromosomes) coexist. The proportion of trisomic cells and where in the body they are concentrated play a large role in how severely the child is affected. A person whose trisomic cells are mostly confined to the placenta or skin, for instance, may fare quite differently from someone whose heart or brain tissue carries the extra chromosome. This variability makes predicting outcomes genuinely difficult.

How the Extra Chromosome Gets There

The underlying cause is a mistake in cell division called nondisjunction, where a pair of chromosomes fails to separate properly when eggs or sperm are being formed. For trisomy 14 specifically, about 83% of cases trace back to an error in the mother’s egg cell, with the remaining roughly 17% originating from the father’s sperm.¹ That split is similar to what researchers see with other trisomies involving acrocentric chromosomes (the group that includes chromosomes 13, 14, 15, 21, and 22). In maternal cases, the error tends to happen during the first stage of egg cell division, while paternal cases more often involve the second stage.

Chromosome 14 belongs to a structural category called acrocentric chromosomes, which have their centromere positioned near one end. These chromosomes are particularly prone to a type of rearrangement known as a Robertsonian translocation, where two acrocentric chromosomes fuse together. When a parent carries a balanced Robertsonian translocation involving chromosome 14, their children face an increased risk of receiving an unbalanced set of chromosomes, potentially leading to trisomy 14 or related conditions.² In these families, the recurrence risk is higher than for someone whose child’s trisomy arose from a random nondisjunction event.

Mosaicism itself usually arises after fertilization. An embryo that starts out with trisomy 14 in all its cells may undergo what is called “trisomy rescue,” where some cells spontaneously lose the extra chromosome during early cell divisions. The result is a body made up of two cell lines: one with the standard 46 chromosomes and one still carrying 47. The timing and location of this rescue event determine which tissues end up with normal versus trisomic cells, and consequently how mild or severe the effects are.

Physical Features and Medical Concerns

Children born with mosaic trisomy 14 share a recognizable pattern of features, though severity varies widely. A detailed review of 15 reported patients found that the following were present in over 90% of cases: growth restriction, a broad nose, ears that appear low-set or unusually shaped, a small jaw, a short neck, and congenital heart disease.³ In males, genital differences including undescended testes were also seen in every case in that series.

Congenital heart defects are among the most medically significant complications. These can range from relatively simple septal defects (holes between heart chambers) to complex structural malformations. One prenatal case showed a combination of a double outlet right ventricle, ventricular septal defect, and pulmonary stenosis alongside severe growth restriction.⁴ Surgical correction of some heart defects is possible; the same review of 15 patients noted a girl with tetralogy of Fallot who showed a marked improvement in health after cardiac surgery.³

Abnormal skin pigmentation is another feature that clinicians look for. A Korean case report described a 17-month-old girl who presented with unusual skin pigmentation along with delayed development, facial differences, and failure to thrive.⁵ The pigmentation differences often follow lines on the skin known as the lines of Blaschko, reflecting where trisomic cells and normal cells settled during embryonic development. These streaky or swirled patterns can sometimes be the first visible clue that mosaicism is present.

Intellectual Development Is Not One-Size-Fits-All

Developmental delay has historically been considered universal in mosaic trisomy 14. Most reported children do show delays in motor skills, speech, and cognitive development, and intellectual disability of varying degree is common. One study contrasting an adolescent girl with mosaic trisomy 14 against a comparison group with Down syndrome found that the girl with trisomy 14 performed below age expectations on most assessments and showed particular weaknesses in language, adaptive functioning, and executive functioning relative to the comparison group.⁶

That said, the picture is not entirely bleak. At least one child with mosaic trisomy 14 showed no evidence of neurodevelopmental delay at age six on formal testing, challenging the earlier assumption that intellectual disability was inevitable.⁷ Because the percentage and distribution of trisomic cells differ from person to person, so do the cognitive outcomes. Families receiving a prenatal diagnosis should understand that published case reports tend to skew toward more severe presentations; milder cases may simply go undiagnosed or unreported.

How Trisomy 14 Is Detected

Prenatal detection often begins with ultrasound findings that raise suspicion. Growth restriction, congenital heart defects, and excessive amniotic fluid (polyhydramnios) are among the flags that prompt further genetic testing. In one reported case, severe polyhydramnios combined with a borderline small jaw led clinicians to order genetic analysis, which confirmed mosaic trisomy 14.⁸

Confirming the diagnosis requires examining the chromosomes directly, usually through amniocentesis. One of the diagnostic challenges is that the level of mosaicism can differ between cultured and uncultured cells from the same sample, and can even change over time. A particularly instructive case involved a pregnancy where initial amniocentesis showed about 13% trisomic cells among cultured amniocytes, but testing of uncultured cells from the same sample showed no trisomy at all. A repeat amniocentesis a few weeks later found the mosaicism had dropped to 7%, and after birth, the infant’s blood and tissue showed no trace of the trisomic cell line.⁹ That child had a favorable outcome, which underscores how tricky it can be to counsel families based on a single test result.

Non-invasive prenatal testing (NIPT), which screens cell-free fetal DNA in the mother’s blood, can flag trisomy 14 as well, but it is a screening test, not a diagnosis. A positive NIPT result for trisomy 14 needs confirmation through amniocentesis or chorionic villus sampling. False positives can occur when the extra chromosome is confined to the placenta rather than present in the fetus itself, a phenomenon known as confined placental mosaicism.

Trisomy Rescue and Uniparental Disomy

When an embryo with trisomy 14 undergoes trisomy rescue and loses the extra chromosome in some or all cells, there is usually no lasting consequence beyond the mosaicism itself. But occasionally, the rescue process introduces a second problem. If the cell happens to lose the sole copy contributed by one parent and retains both copies from the other parent, the result is uniparental disomy, or UPD. For chromosome 14, this matters because the chromosome carries imprinted genes whose activity depends on which parent they were inherited from.

Trisomy rescue is considered the most frequent mechanism producing UPD of chromosome 14, and the initial trisomy in these cases often stems from a Robertsonian translocation involving chromosome 14, though maternal meiotic nondisjunction or mitotic errors after fertilization can also be responsible.² The clinical significance depends entirely on whether both retained copies come from the mother or the father, because the two scenarios produce very different conditions.

Temple Syndrome and Kagami-Ogata Syndrome

When both copies of chromosome 14 come from the mother (maternal UPD14), the resulting condition is called Temple syndrome. It is considered relatively mild as imprinting disorders go.¹⁰ Children with Temple syndrome typically present with low muscle tone in infancy, early feeding difficulties, growth restriction, and notably early puberty. The condition is thought to stem from the absence of activity of a gene called DLK1, which is normally expressed only from the paternal copy of chromosome 14. A study of 32 Japanese patients with Temple syndrome found that the vast majority had maternal UPD as the underlying mechanism, with smaller numbers caused by epimutations or tiny deletions in the 14q32.2 imprinted region.¹¹

When both copies come from the father (paternal UPD14), the picture is much more severe. This condition, called Kagami-Ogata syndrome, involves a distinctive set of features including a characteristic facial appearance with full cheeks and a protruding philtrum, a small bell-shaped chest with ribs that angle outward in a pattern sometimes described as resembling a coat hanger on X-ray, abdominal wall defects, an enlarged placenta, and polyhydramnios during pregnancy.¹² The small, rigid chest can restrict breathing and is a major source of early morbidity. Kagami-Ogata syndrome is thought to result from overexpression of a gene called RTL1, which is normally kept in check by the maternally expressed RTL1as gene on the opposite chromosome.

These two syndromes illustrate a broader principle: for chromosome 14, it is not just the number of copies that matters but also which parent contributed them. A child could have the correct total of two chromosomes yet still have a clinically significant condition if both came from the same parent.

What Research in Animal Models Has Revealed

Much of the detailed understanding of how chromosome 14’s imprinted genes function comes from mouse studies. The corresponding imprinted region in mice sits on chromosome 12 and contains an equivalent cluster of genes, collectively called the Dlk1-Dio3 cluster.¹³ Researchers have been able to selectively turn individual genes in this cluster on or off in mice, mapping which gene drives which aspect of the human conditions. This work has confirmed, for example, the central role of Rtl1 overexpression in the skeletal and placental features of Kagami-Ogata syndrome, and the role of Dlk1 loss in the growth restriction and early puberty seen in Temple syndrome.

Mouse studies involving induced pluripotent stem cells have also shown that cells with trisomy 14 (or its mouse equivalent) can have a growth advantage over normal cells in culture, which may help explain why the trisomic cell line persists in mosaic individuals rather than being outcompeted and disappearing over time.¹⁴ This finding is a reminder that mosaicism is not a static snapshot. The balance between normal and trisomic cells can shift as the person grows, which adds another layer of unpredictability to the condition’s course.

Living with Mosaic Trisomy 14

Because the condition is so rare, there are no standardized treatment guidelines. Management is driven by the individual child’s specific medical issues. Congenital heart defects may require surgical repair. Feeding difficulties in infancy often need specialized support, and many children benefit from early-intervention therapies addressing motor and speech development. Growth monitoring is important given that failure to thrive is nearly universal in reported cases.

The long-term outlook varies enormously. Some children face life-threatening complications from severe heart defects or other organ involvement in early life, while others survive into adolescence and adulthood with varying degrees of intellectual disability and ongoing medical needs. The child described earlier who showed no developmental delay at age six is a reminder that the published literature does not capture the full spectrum, and that lower levels of mosaicism may produce much milder outcomes than the typical case report suggests.⁷

For families navigating a new diagnosis, connecting with genetic counselors who have experience with rare chromosomal conditions is valuable. Genetic counselors can help interpret the specific level and distribution of mosaicism, discuss what is and is not known about prognosis, and guide decisions about monitoring and intervention. Because each case is essentially unique, generalized predictions are unreliable, and personalized assessment is the only honest approach.

Confined Placental Mosaicism and False Alarms

One scenario worth understanding is confined placental mosaicism, where the trisomic cell line exists in the placenta but not in the fetus. This can happen because the placenta and the embryo diverge from different cell populations very early in development. If trisomy rescue occurs in the cells that become the embryo but not in those that become the placenta, the placenta will test positive for trisomy 14 while the baby is chromosomally normal.

This matters in practice because both chorionic villus sampling and NIPT sample placental tissue or placental DNA, not fetal tissue directly. A finding of trisomy 14 on either of these tests does not necessarily mean the fetus is affected. Follow-up amniocentesis, which samples the amniotic fluid more closely reflecting the fetus, is needed to clarify. Even then, as the case described earlier demonstrated, results can differ between cultured and uncultured amniocyte samples and can change between the first and second amniocentesis.⁹ The anxiety this creates for families is real, and it is worth knowing that a positive screen does not always mean a poor outcome.

Confined placental mosaicism can also cause complications for the pregnancy itself, including growth restriction, even when the fetus’s own chromosomes are normal. The placenta’s function can be impaired when a significant fraction of its cells carry an abnormal chromosome complement. So even in the best-case scenario of a chromosomally normal baby, extra monitoring during pregnancy may still be warranted.

How Trisomy 14 Compares to Better-Known Trisomies

Most people have heard of trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). These are the three autosomal trisomies that can result in live births without mosaicism, though survival with trisomies 18 and 13 is often limited. Trisomy 14 belongs to a different category: trisomies that are lethal in their complete form and only survive as mosaics. Other chromosomes in this group include trisomies 8, 9, 16, and 22.

What sets chromosome 14 apart is the imprinting dimension. Because chromosome 14 carries genes whose expression depends on parental origin, the consequences of having an extra copy or losing the wrong copy are more complex than with a chromosome where both parental copies behave identically. This imprinting layer means that genetic counseling for trisomy 14 has to consider not just the mosaicism but also the possibility that trisomy rescue has produced UPD, which would require its own clinical evaluation and management. Testing for UPD is not automatic and may need to be specifically requested when trisomy 14 mosaicism is diagnosed.