We are here to help. Fraternal twins are created when two separate eggs are fertilized by two separate sperm. While some fraternal twins look stunningly alike, they are nevertheless the product of two separate fertilizations. Identical twins, however, are created when one egg is fertilized by one sperm, and sometime after fertilization, this single egg miraculously splits, forming two distinct embryos with the exact same genes. Therefore, appearance alone cannot differentiate between fraternal and identical twins.
Identical twins are also always the same sex.
No one knows exactly what makes the egg split. It appears to be random, occurring approximately 1 in every 250 pregnancies. Monoamniotic twins are always identical. When the egg splits is an important fact. To understand why, you'll need to understand a little about the biology of the forming embryo.