Conjoined twins - the facts

The chances of a woman having conjoined, or Siamese twins, are extremely rare, affecting one in every 100,000 births.

Siamese twins originate from a single fertilised egg and are therefore always identical and of the same sex. Identical twins are created when a fertilised egg splits in two.

But in the case of Siamese twins, that process stops before it is complete - leaving a partially separated egg which develops into a conjoined foetus. Between 40 and 60 per cent of conjoined twins are stillborn and about a third of live births do not survive more than one day.

The overall survival rate of conjoined twins is somewhere between five and 25 per cent. If doctors operate to separate twins, the best chances of survival are among those who do not share major organs.

The fact that Natasha and Courtney share a heart means it would be impossible for both to survive after separation. Siamese twins can be joined in three ways - at the chest or upper abdomen, the hips and legs, or the head.

Over the years, survival rates have improved as a result of more accurate imaging studies and better anaesthetic and operative techniques.