Scientists stunned by discovery of 'semi-identical' twins

Boy and girl, now four, are only the second case of 'sesquizygotic' twins recorded

A pair of twins have stunned researchers after it emerged that they are neither identical nor fraternal - but something in between.

The team say the boy and girl, now four years old, are the second case of semi-identical twins ever recorded, and the first to be spotted while the mother was pregnant.