What Is Trisomy 13?

You’ve likely heard of Down syndrome, also known as trisomy 21, but you may be less familiar with trisomy 13. Trisomy 13 is a much less common condition than Down syndrome, but like Down syndrome, Trisomy 13 is caused by having an extra copy of a chromosome – in this case, an extra copy of chromosome 13. Trisomy 13 is also sometimes referred to as Patau syndrome. 

The extra genetic instructions from the additional copy of chromosome 13 causes significant developmental issues in babies with trisomy 13. Children with this diagnosis frequently have physical differences (also referred to as “birth defects”) in different organ systems. Examples of these differences include heart defects, brain and spinal cord abnormalities, extra fingers and/or toes, and clefting of the lip. Approximately 1 in 16,000 babies is born with Trisomy 13. Because miscarriage and stillbirth are common with trisomy 13, the number of pregnancies with this condition is higher than the number of babies born with the condition. A parent of any age can have a child with trisomy 13, but the chance of having a baby with trisomy 13 increases with increasing maternal age.

What are the features of trisomy 13? 

The extra chromosome 13 causes distinct physical differences or birth defects that may be seen on ultrasound scans during pregnancy or observed at birth. 

While not all babies with trisomy 13 will have all of these findings, common features include the following: 

• Small size and poor growth
• Heart defects
• Extra fingers or toes (“polydactyly”)
• Clenched hands 
• Cleft lip and/or cleft palate
• Small head size with small eyes (“microphthalmia”) and small lower jaw (“micrognathia”)
• Close-set eyes – in some cases, the eyes may fuse together into a single eye
• Low-set ears
• Undescended testicles (“cryptorchidism”)
• Decreased muscle tone (“hypotonia”)
• Umbilical hernia and/or inguinal hernia
• Brain and spinal cord abnormalities, including fusion of the brain hemispheres (“holoprosencephaly”) and non-closure of the spinal cord (“spina bifida”)
• Seizures
• Intellectual disability

If a pregnancy is diagnosed with or suspected to have trisomy 13, close monitoring is performed via ultrasound surveillance, and delivery at a high-risk medical center is typically recommended. Fewer than 5% of all live-born babies born with trisomy 13 will not live past 1 year of age due to the severity and complexity of their physical and intellectual challenges. Those who live past infancy will have significant intellectual disability and require specialized medical care; however, each individual with trisomy 13 is unique, and genetic testing alone is not able to predict the exact course of their condition. 

What causes trisomy 13? 

The majority of cases of trisomy 13 occur by chance, meaning the condition does not typically run in families. Most of the time, trisomy 13 is caused by having three copies of chromosome 13 in each cell of the body rather than the two copies seen in most people. The extra copy of chromosome 13 can come from either the genetic mother or the genetic father. Typically, each parent passes on one copy of each chromosome (23 chromosomes total), so that when the egg and the sperm come together, the baby receives 46 chromosomes total with one copy of each chromosome being inherited from each parent (23 total pairs). In trisomy 13, either the genetic mother’s egg cell or the genetic father’s sperm cell contributes an additional copy of chromosome 13. This means the baby receives 47 chromosomes rather than the typical 46. 

Less commonly, trisomy 13 may be due to a chromosome rearrangement known as a translocation. A translocation means that all or part of a chromosome is attached (translocated) to another chromosome. If a parent “carries” a translocation involving chromosome 13 (meaning, their chromosomes are rearranged but still “balanced” with respect to the total amount of genetic information”), they could pass on extra chromosome 13 material to their offspring. In these cases, a baby might inherit a translocated piece of chromosome 13 in addition to the two normal copies of chromosome 13 – resulting in three total copies of chromosome 13. However, most genetic parents of babies with trisomy 13 do not carry a translocation, and the condition typically occurs due to chance alone rather than running in the family. 

In rare cases, the extra chromosome 13 may be present in only some cells of the body rather than every cell. This is called mosaic trisomy 13. The severity of the symptoms of mosaic trisomy 13 depends on the proportion of cells with the extra chromosome and the tissue types or organ systems in which the extra chromosome is present.

How is trisomy 13 diagnosed? 

Trisomy 13 may be suspected during pregnancy based on ultrasound findings or the results of prenatal genetic screening tests such as non-invasive prenatal screening (NIPS). NIPS works by analyzing small fragments of DNA circulating in a pregnant person’s blood. These fragments are called cell-free DNA (cfDNA) because, unlike most DNA found inside a cell’s nucleus, these fragments are free-floating and not within cells. During pregnancy, the pregnant parent’s bloodstream contains a mix of cfDNA from her own cells and cfDNA from the placenta. Because the placenta and the fetus originate from the same group of cells (embryo), placental DNA is usually identical to fetal DNA. Therefore, NIPS can be used to screen for chromosomal conditions in the fetus. Importantly, screening tests for trisomy 13 are designed to identify pregnancies that may be at increased risk for this condition. Still, a screening test cannot provide a definitive diagnosis, and false positives or false negatives can occur. Therefore, additional genetic testing is recommended to confirm a diagnosis of trisomy 13 after a high-risk screening result. 

To diagnose trisomy 13, a genetic test called a karyotype can be done to assess the number and arrangement of chromosomes in an individual, and a diagnosis of trisomy 13 is made when testing reveals the presence of a third copy of chromosome 13. 

During pregnancy, trisomy 13 can be diagnosed via prenatal diagnostic procedures such as chorionic villus sampling (CVS) or amniocentesis. CVS is done in the first trimester and involves taking a sample (biopsy) of tissue from the placenta using either a catheter through the cervix (transcervical CVS) or a slender needle through the abdomen (transabdominal CVS). This tissue can then be analyzed to look at the placenta’s chromosomes through a test called a karyotype, also referred to as chromosome analysis. Amniocentesis is a procedure typically done in the second trimester, during which a small amount of amniotic fluid is drawn from the amniotic sac using a long, slender needle. A karyotype can be performed using the fetal cells in the amniotic fluid to diagnose chromosome conditions such as trisomy 13. Both CVS and amniocentesis are considered invasive procedures and are associated with pregnancy risks, including a slight risk of pregnancy loss. Research on the outcomes of these procedures over the past several decades suggests that for the average pregnant person, there is approximately a 1 in 300 to 1 in 1,000 chance for miscarriage following CVS or amniocentesis.   

In addition to karyotype, another type of genetic analysis called chromosomal microarray analysis, or CMA, can be performed to look for smaller chromosomal segments that may be extra or missing. These are called microduplications and microdeletions, respectively. Chromosomal microarray analysis can be helpful for detecting cases of partial trisomy 13, where there is an extra piece of only part of chromosome 13 rather than an extra entire chromosome 13. 

After birth, karyotype or chromosomal microarray analysis can be performed on a simple blood sample to diagnose trisomy 13. Chromosomal microarray may be done in cases where a newborn is found to have a variety of physical differences suggestive of a genetic syndrome, but it is unclear which syndrome is most likely, and the healthcare provider wants to test for as many chromosomal abnormalities as possible. 

How is trisomy 13 treated? 

Specialists such as neonatologists, geneticists, nephrologists, neurologists, surgeons, genetic counselors, and social workers typically work together to care for the complex needs of an infant with trisomy 13. Unfortunately, there is no cure for trisomy 13, and medical support at birth typically focuses on stabilization and minimizing the pain or suffering of the baby. Families facing a diagnosis of trisomy 13 will work closely with the healthcare team to come up with a treatment plan that’s right for their situation. They may consider factors such as maximizing the baby’s quality of life and making them as comfortable as possible with minimal medical intervention (“comfort care”). Other families may choose to pursue more aggressive interventions, such as surgeries to repair heart defects or cleft lips. Even with surgical intervention and specialized care at birth, babies with trisomy 13 will have significant physical and intellectual disabilities and a shortened life span with a typical life expectancy of less than 1 year. Because of the many challenges these babies face and the intense care they require, support from family members and loved ones is crucial. 

Where can I find support or learn more about trisomy 13? 

A diagnosis of trisomy 13 can be the beginning of a challenging journey where families face difficult decisions and experience many emotions, including shock, anger, disappointment, uncertainty, and grief. For some families, connecting with others in similar situations may provide them with a sense of community and comfort. However, every family's journey with trisomy 13 is unique, and some may wish for space to heal from their experience before connecting with others.

To learn more about the medical implications of trisomy 13, visit the MedlinePlus website, or to communicate with families who have received a diagnosis of trisomy 13, visit SOFT - the Support Organization for Trisomy 18, 13, and Related Disorders at https://trisomy.org/.

- Authored by Kelly Miller, MS, LCGC | Genetic Counselor

Please note: JunoDx.com and the materials and information it contains are not intended to, and do not constitute medical or other health advice or diagnosis and should not be used as such. You should always consult with a qualified physician or health professional about your specific circumstances.