There is a larger incidence of female patients with Trisomy 18 that occurs mostly because of non-disjunction in the meiosis of oocytes. Why does this happen?
According to this article, the ratio of affected females/males differs when you look at liveborn or prenatal infants, which indicates a prenatal selection against males (i.e. more males are spontaneously aborted).
The article links to more references for further reading.