Patau syndrome, in 75%-90% of cases, is the result of the presence of a whole extra (third) copy of chromosome 13, hence its alternative name of trisomy 13 (or simple trisomy 13).
Patau syndrome is caused by a chromosomal translocation in 5%-10% of cases, and to mosaicism (whereby only some cells have the extra copy of chromosome 13) in 5%. In occasional cases, it is only a part of chromosome 13 that is extra (partial trisomy 13).
About 90% of cases of simple trisomy 13 are caused by non-disjunction in maternal meiosis, and the likelihood of occurrence rises with increasing maternal age.
For parents of a child with simple trisomy 13, the risk of recurrence for trisomy 13 or for the other major chromosomal abnormalities is usually very low (typically 1% unless the maternal age-dependent risk is already above 1%).
If one parent is a carrier of a balanced translocation, the risks are higher.