Trisomy 18

Inheritance

Is Trisomy 18 inherited?

Most often, trisomy 18 is not inherited. Trisomy 18 most often occurs because of a random mistake in the division of egg or sperm cells. However, trisomy 18 can be inherited if a parent has a rearrangement of chromosome material that involves chromosome 18. This rearrangement can also be called a “balanced translocation”. Humans generally have 46 chromosomes (23 pairs) in every cell in their body. We label them chromosome 1, chromosome 2, chromosome 3, etc. Humans generally have two copies of every chromosome, including chromosome 18. Sometimes bits of chromosomes can swap places. For example the top of one chromosome 2 and the top of one chromosome 18 switch places. The swap is called a translocation or a rearrangement. A person with that particular translocation would have a balanced translocation, because they aren’t missing any genetic information. That person’s children would have an increased chance to inherit a chromosome abnormality though, like partial trisomy 18. Analyzing a parent’s chromosomes can determine whether a parent has a balanced translocation. Balanced rearrangements are identified in about 1/1000 individuals, usually through the birth of a baby with an unbalanced chromosome condition.

References
  • http://rarediseases.org/rare-diseases/trisomy-18-syndrome/
  • http://ghr.nlm.nih.gov/condition/trisomy-18
  • Int J Dev Biol. 2012;56(10-12):841-52. doi: 10.1387/ijdb.120141ue.Oocyte ageing and its cellular basis.Eichenlaub-Ritter U1.
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How is Trisomy 18 most often caused?

What is the chance to have another child with Trisomy 18?

Does the chance to have a child with Trisomy 18 increase as a woman ages?

How is Trisomy 18 most often caused?

Trisomy 18 most often occurs randomly as sperm and eggs are created, most commonly due to a process called nondisjunction. Humans generally have 46 chromosomes (23 pairs) in every cell in their body. When a woman’s eggs are made, each egg generally has one copy of each of the 23 chromosomes. The same is true for sperm. The 23 copies from the sperm and the 23 copies from the egg then combine to create a person with 46 chromosomes (23 pairs). When nondisjunction happens, an egg or a sperm is created with 2 or 0 copies of a chromosome instead of the usual 1. If an egg with two chromosome 18’s combines with a sperm with 1 chromosome 18, the baby will have trisomy 18.

References
  • http://rarediseases.org/rare-diseases/trisomy-18-syndrome/
  • http://ghr.nlm.nih.gov/condition/trisomy-18
What is the chance to have another child with Trisomy 18?

If neither parent has a chromosome rearrangement, the chance to have a second baby with trisomy 18 is thought to be low, but not impossible. If there is a suspicion of trisomy 18 in a baby, it is important to confirm this with genetic testing so the most accurate recurrence risk (the chance that the couple will have another baby with the same condition) can be provided. There is around a 1% chance to have a second child with trisomy 18 or another trisomy such as trisomy 21 or trisomy 13. The chance to have a child with trisomy 18 increases as a women ages; even though trisomy 18 is rare, it is seen more frequently in babies born to women in their late 30s and early 40s, as compared to babies born to younger women. Prenatal testing is available and is generally offered to interested parents through their healthcare providers caring for them during pregnancy. Prenatal genetic counselors can also provide individualized information about the possibility of having another pregnancy with a chromosome problem.

References
  • http://ghr.nlm.nih.gov/condition/trisomy-18
  • http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3520824/
  • Cereda, A and Carey, J.C. The trisomy 18 syndrome. Orphanet J Rare Dise. 2012; 7:81
Does the chance to have a child with Trisomy 18 increase as a woman ages?

The chance to have a child with trisomy 18 does increase as a woman ages. Overall, more babies with trisomy 18 are born to women under the age of 35, but the chance to have a child with trisomy 18 is greater in mothers who are over 35 years old. A prenatal or reproductive genetic counselor can share the specific risk for a woman considering pregnancy. The National Society of Genetic Counselors can help in locating a genetic counselor in your area, Search for a genetic counselor here

References
  • http://rarediseases.org/rare-diseases/trisomy-18-syndrome/
  • Clin Exp Obstet Gynecol. 2016;43(2):287-90.A rare occurrence of three consecutive autosomal trisomic pregnancies in a couple without offspring.Burada F, Sosoi S, Iliescu D, Ioana M, Cernea D, Tudorache S.
  • Orphanet J Rare Dis. 2012 Oct 23;7:81. doi: 10.1186/1750-1172-7-81.The trisomy 18 syndrome.Cereda A1, Carey JC.

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