What is the cause of edward's syndrome?

Defective transfer. When eggs/sperm form they go through a process where the pairs of chromosomes split.Each resulting egg or sperm should only have one of the pair. When /if an egg or sperm fails to drop one or part of the chromosome 18, it will have excessive material once conception occurs.That extra material interferes with normal development.
Trisomy 18 . Edwards syndrome (also known as trisomy 18 (t18)) is a genetic disorder caused by the presence of all or part of an extra 18th chromosome. This genetic condition almost always results from nondisjunction during meiosis. It is named after john h. Edwards, who first described the syndrome in 1960. It is the second most common autosomal trisomy, after down's syndrome, that carries to term.