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. One of the first things to form is the yolk sac, which is the beginning of a placenta. After the yolk sac has formed, the chorionic sac forms. After that, the amniotic sac.