Nobody, not even the doctor can predict, how a woman's egg would be fertilized or how they will develop. Giving birth to twin is actually not so much of a rarity these days but giving birth to Siamese twins or conjoined twin are an extreme rare occurrence.
Most of the times, the lives of these children are very sort being unable to function properly and there have been incident were more than half the known births of conjoined twins have resulted in stillborn babies.
They are actually eight types in which twins might be conjoined but among these three are extremely rare types.