Genetics Northern California

Trisomy 13

Trisomy 13 is a rare chromosome abnormality, affecting about one in every 5,000 to 10,000 babies. This condition includes both severe mental retardation and physical birth defects. Most babies with trisomy 13 are stillborn or survive just a short period of time after birth. Less than one out of every three babies born with trisomy 13 will live to six months of age, and only one in ten survive to their first birthday. A very small minority of children with trisomy 13 can live for an extended length of time.  

The degree of mental retardation in children with trisomy 13 is usually severe to profound. Even children who survive long term will very rarely walk or have speech. In addition to mental retardation, babies with trisomy 13 often have physical birth defects. The most common of these are a cleft lip (when the upper lip does not form properly), eyes which are small and closer together than is normal, heart defects, kidney abnormalities, and extra fingers or toes. Sometimes the baby's brain does not form properly, and instead of having the usual two distinct hemispheres (halves), there is only a single continuous structure. This is called holoprosencephaly. Often this type of brain abnormality leads to death in the newborn period.

Trisomy 13 is caused by an extra chromosome, specifically an extra copy of chromosome number 13.  Chromosomes are the packages of genetic material which provide the instruction for growth and development. Normally, the chromosomes are paired, one in each pair from the mother, and one from the father.  The presence of three copies of chromosome13, instead of two, interferes with the instructions for growth and development in the baby.  

The extra chromosome 13 is present at the time of conception. It occurs as a random event when either the egg or sperm is being formed. There is nothing known to either cause or prevent this from happening. As a woman’s age increases, she is more likely to have a baby with a chromosome abnormality, but these types of birth defects can happen at any age.

Rarely, a baby has trisomy 13 due to a rearrangement of part of chromosome 13 with another chromosome. These cases are called "translocations" and can run in families, although most of the time they do not.

A chromosome study, either by prenatal testing or by blood test after birth, can accurately diagnose trisomy 13. Testing can also show whether the baby has the rare "translocation" form of trisomy 13. 

Sometimes an abnormal ultrasound provides the first indication that the baby has a problem. In order to confirm a diagnosis of trisomy 13, cells from the fetus are needed for evaluation. Trisomy 13 can be diagnosed prenatally by having CVS or amniocentesis.  In other cases a baby with trisomy 13 may be diagnosed shortly after birth. If facial features, a heart defect or other problems raise the possibility of trisomy 13, a blood chromosome test will be done to confirm the diagnosis.

When a baby is born with trisomy 13, parents usually need to make some difficult decisions about whether to use medical interventions to prolong the baby's life, or whether to simply give the baby supportive care to make him or her as comfortable as possible. You can receive help and support in this process from the baby's doctor, the nursery social worker, hospital chaplain and your genetic counselor.

Other Information
If you have learned during your pregnancy that your baby has trisomy 13 and are trying to decide whether or not to continue your pregnancy, you will probably not find many sources of information on this condition. There is an organization called SOFT (Support Organization for trisomy 18, 13, and Related Disorders) which is composed of parents who have children with trisomy 13 and other related conditions as well as medical professionals with an interest in caring for children with these conditions. These parents are strong advocates for their children and emphasize the positive aspects of raising a child with trisomy 13. Parents who have a child with trisomy 13 have found this group to be a great support


Genetics Home Reference: Trisomy 13 - General information about trisomy 13 provided by the National Library of Health. Also includes links to related sites.

SOFT - Support group for parents who have had or are expecting a child with a chromosome disorder (especially trisomy 18 and trisomy 13).