Trisomy 18 can occur through a parental non-disjunction at either the first or the second meiotic division, or more rarely, a parental translocation. Where there is straightforward trisomy, the additional chromosome is probably inherited from the mother in over 90% of cases.
The risk of recurrence of this condition is low. Antenatal diagnosis by amniocentesis or chorion villus sampling is available.
Mosaicism also occurs, and probably accounts for most long-term survivors.
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