There are conjoined twins which are formed, a single egg develops into 1 baby. Sometimes a single egg divides in half 1 to 2 weeks after it's fertilized. This creates a set of identical twins. The exact cause of conjoined twinning is not known.
There are non identical twins.Non-identical twins form from two completely separate eggs which are fertilised by two completely separate sperm. These fraternal twins are no more alike than any other siblings in a family with the same biological mother and father. Because identical twins share the same genes, they are always the same gender.
