Why do multiples occur?

Multiple pregnancies occur when either more than one egg is released for fertilisation, or when one egg is fertilised and subsequently splits.

In identical multiples, the egg splits shortly after fertilisation (within 2 weeks). Identical triplets and higher order multiples occur when one or more of these eggs splits again.  Identicals will be of the same sex and have the same genetic make-up as one another.

In non-identical multiples, more than one egg is fertilis....