The following information may help to address your question:
What causes trisomy 13?
Most cases of trisomy
13 are caused by random events during the formation of eggs or sperm in healthy parents (prior to conception). Trisomy 13 is typically due to having three full copies of chromosome 13
in each cell
in the body, instead of the usual two copies. This is referred to as complete trisomy 13 or full trisomy 13. The extra genetic material
present disrupts the normal course of development, causing the characteristic features of the disorder.
Trisomy 13 can also occur when part of chromosome
13 becomes attached (translocated) to another chromosome during the formation of eggs or sperm, or very early in fetal development. This is referred to as translocation
trisomy 13. People with this type of trisomy 13 have the normal two normal copies of chromosome 13, plus an extra copy of chromosome 13 that is attached to another chromosome. Translocation trisomy 13 can be inherited
. Approximately 20% of cases of trisomy 13 are caused by translocation trisomy 13.
In rare cases, only part of chromosome 13 is present in three copies in each cell (rather than the full chromosome); this is called partial trisomy 13. In other rare cases, a person has an extra copy of chromosome 13 in only some of the body's cells; this is called mosaic
trisomy 13. The severity of mosaic trisomy 13 depends on the type and proportion of cells that have the extra chromosome.
Last updated: 4/25/2016
Is trisomy 13 inherited?
Most cases of trisomy 13 are not inherited and result from random events during the formation of eggs and sperm in healthy parents. An error in cell division called nondisjunction
results in an egg or sperm cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 13. If one of these contributes to the genetic makeup of a child, the child will have an extra chromosome 13 in each cell of the body.
For a woman who has never had a pregnancy or child with a trisomy, the risk to have a pregnancy with a chromosome abnormality
would typically be her age-related risk - the risk increases with maternal age. For a woman who has had an affected pregnancy or child in the past, the recurrence risk is generally quoted as about 1% or
the maternal age-related risk - whichever is higher at the time of pregnancy.
On the other hand, translocation trisomy 13 can be inherited. An unaffected parent can have a rearrangement of genetic material between chromosome 13 and another chromosome. These rearrangements are called balanced translocations
because there is no extra or missing genetic material from chromosome 13. A person with a balanced translocation involving chromosome 13 has an increased chance of passing extra material from chromosome 13 to his/her children.
People interested in learning about their personal risk to have a child with a chromosome abnormality should speak with their health care provider or a genetics professional.
Last updated: 4/25/2016
If a previous pregnancy was affected by trisomy 13, what is the chance of having another affected pregnancy?
The exact risk to have more than one pregnancy affected by trisomy 13 may depend on the underlying cause of trisomy 13 in the first affected fetus or child. The recurrence risk varies depending on whether the initial trisomy 13 occurred because a parent carries a balanced translocation
involving chromosome 13, or because random events prevented two copies of chromosome 13 from separating during the formation of an egg or sperm (called nondisjunction
Because trisomy 13 is rare and usually occurs due to a random error, it is generally very unlikely to have more than one affected pregnancy or child. However, it is difficult to determine what the exact chance is.
One study estimated that there is about a 1 in 333 (0.3%) chance for women who have had a pregnancy affected by trisomy 13 to have a second affected pregnancy. This study also suggested there is approximately a 1 in 111 (0.9%) chance for a future pregnancy to be affected by another type of trisomy, such as trisomy 21 or trisomy 18.
Women who have had an affected pregnancy and are considering future pregnancies are encouraged to speak with their health care provider or a genetics professional to discuss personal recurrence risks and management of future pregnancies.
Last updated: 3/17/2016
Is genetic testing available for someone who had a previous pregnancy affected by trisomy 13?
While most cases of trisomy 13 occur randomly, a few cases are due to the presence of a translocation
involving chromosome 13 in a parent. Parents who are at risk to have a translocation due to their family history
can have a blood test called a karyotype
, which can determine if a translocation is present.
Prenatal testing or screening
(such as maternal blood screening
, fetal ultrasound
, chorionic villus sampling
, or amniocentesis
) is also available to determine if a current pregnancy is at risk for, or is affected by, trisomy 13 or other chromosome disorders.
People with a family history of trisomy 13 who are interested in learning about genetic screening
or testing for themselves or family members are encouraged speak with a genetic counselor
or other genetics professional.
Last updated: 3/17/2016
How can I find a genetics professional in my area?
Last updated: 7/15/2016
We hope this information is helpful. We strongly recommend you discuss this information with your doctor. If you still have questions, please
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