How monozygotic twins form
Monozygotic twins, also known as identical twins, develop from one egg fertilized by one sperm. The frequency of monozygotic twins is around 1 in 250 pregnancies without assisted fertility.
Identical twins begin as a single egg that has been fertilized by a single sperm. From that beginning, the egg will split into two. Depending on how early the split happens, monozygotic twins may or may not share the same placenta (monochorionic) and same amniotic sac (monoamniotic).