Identical twins are babies that are formed from the same fertilised egg, as opposed to non-identical twins which are formed from two fertilised eggs.
The creation of identical twins - or monozygotic twins - happens when the fertilised egg splits into two halves. This leads to the development of two individual embryos.
Because their origin is from one fertilised egg, they will share exactly the same DNA and will always be the same sex.
It is not known why the fertilised egg splits in two. It is not genetic and, for that reason, identical twins do not run in families. It is therefore believed to be a malfunction of the biological process.
Identical twins represent around one third of twin births - in fact, there is a 1 in 3000 chance of giving birth to identical twins.
This statistic is consistent worldwide - there is no single influencing factor such as race or maternal age that will result in an increased chance of identical twin birth. Additionally, fertility treatment is not believed to increase the likelihood of having identical twins.
Despite their shared DNA, identical twins are individuals and their physical looks, behaviours and traits will always vary, making it possible to tell identical twins apart. This also goes for fingerprints - even these are unique to each identical twin.