Identical twins are the result of splitting the fertilized egg into two halves.
However, in rare cases, the egg may divide before fertilization. If different sperm fertilize the egg, the born baby is called a semi-identical twin.
Although they are still identical twins, separate fertilization means they are closer to fraternal twins. However, fraternal twins share approximately 50% of the same DNA, while semi-identical twins have 75% of the same DNA.