Trisomy 18

In most cases, trisomy 18 is caused by having 3 copies of chromosome 18 in each cell in the body, instead of the usual 2 copies. The extra genetic material from the 3rd copy of the chromosome disrupts development, causing the characteristic signs and symptoms of the condition.

About 5% of people with trisomy 18 have 'mosaic trisomy 18' (when there is an extra copy of the chromosome in only some of the body's cells). The severity of mosaic trisomy 18 depends on the number and locations of cells with the extra copy.

Very rarely, an extra piece of chromosome 18 is attached to another chromosome; this is called translocation trisomy 18, or partial trisomy 18. If only part of the long (q) arm of chromosome 18 is present in 3 copies, the features may be less severe than in people with full trisomy 18.

Most cases of trisomy 18 are not inherited and occur randomly due to errors in the formation of eggs or sperm. If an egg or sperm gains an extra copy of chromosome 18 during cell division and contributes to a pregnancy, the embryo will have an extra chromosome 18 (trisomy) in each cell of the body.

Mosaic trisomy 18 (when some body cells have trisomy 18 and some have a normal chromosome make-up), is also typically not inherited. Mosaic trisomy 18 is also due to an error in cell division, but the error occurs early in embryonic development. About 5% of affected people have a mosaic form of trisomy 18.

Partial trisomy 18 (when only part of chromosome 18 is present in 3 copies) can be inherited. An unaffected parent can carry a rearrangement of genetic material between chromosome 18 and another chromosome. This rearrangement is called a balanced translocation because there is no extra or missing genetic material. However, a person with a balanced translocation has an increased risk with each pregnancy to have a child with trisomy 18.

Most cases of trisomy 18 occur as random events during the formation of reproductive cells (eggs and sperm). The risk of recurrence of trisomy (21, 13 or 18) in families of an individual with trisomy 18 is around 1%. However, in families in which trisomy 18 is caused by a translocation, the recurrence risk is higher if one of the parents is a carrier of a balanced translocation. Parents who have a child with translocational trisomy 18 and want additional children are encouraged to have chromosome studies due to this increased risk. The risk to siblings of individuals with trisomy 18 depends, in large part, on the genetic status of the parents, as translocation cases can be passed through families with carriers being asymptomatic. Genetic studies (chromosome analysis) can determine if translocations are present.

We recommend that you discuss this information and your concerns with a genetics professional.

The recurrence risk for trisomy 18 depends on the type of trisomy 18 in an affected family member (full, mosaic, or partial trisomy 18).

The recurrence risk for a family with a child with full trisomy 18 is usually stated as 1% (1 in 100). Therefore, the vast majority of parents with an affected fetus or child go on to have normal children. Parental mosaicism has been reported in a few cases, and recurrence of different trisomies (involving a chromosome other than 18) has also been reported. Experiences have suggested that the recurrence risk seems to be less than 1%, but higher than the maternal age-related risk. The recurrence risk for mosaic trisomy 18 is also thought to be relatively low.

The recurrence risk in families with a child with partial trisomy 18 may be higher than that for full trisomy 18, but the exact risk depends on whether one of the parents has a chromosomal rearrangement (such as a balanced translocation).

People with questions about the recurrence risk of trisomy 18 for themselves or family members are encouraged to speak with a genetics professional.