Twins are born when two babies grow in their mother's womb at the same time. During fertilisation, a single sperm usually fertilises a single egg, which then starts to grow into an embryo, by dividing into two cells, then four, and so on. Occasionally, one embryo splits into two that then grow into genetically identical individuals - clones. Non-identical twins develop if two eggs are fertilised at the same time, by two separate sperm. They have no more genetic similarity than brothers and sisters.