Because they come from the same egg, identical twins are always the same sex according to the National Health Service. Fraternal twins are from two separate eggs, which means that they can be different sexes.
Identical twins are developed from the same egg, which splits after it is fertilized. The two resulting embryos have the exact same genes. A woman is more likely to have twins if she has a history of twins in her family, although many identical twins are random.
Fraternal twins, on the other hand, are often hereditary. They are developed when two eggs are released and fertilized during the same menstrual cycle. They have no more similar characteristics than siblings, so they do not share exactly the same genes and often do not look alike.
The number of twins born across the world has increased significantly in the past several decades because of an increased use in fertility drugs.