The following information may help to address your question:
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.