Question: what makes some twins conjoined when they are born?

Esther Muthumbi answered on 11 Jul 2017:

Usually, one egg is fertilised by one sperm forming one embryo which develops into a singleton pregnancy and birth.
Sometimes, the fertilised egg splits into two forming identical twins. The splitting usually occurs early (8-12 days) and then the organ development begins.

The exact cause of conjoining is not known but the theories are:
1. The splitting may occur late and does not split completely
2. The egg divides completely and then fuses again

The occurrence of conjoined twins is rare and thus difficult to study what the risk factors for it are, but it really is an interesting question