Twins are either identical or fraternal and are always present within the same pregnancy. The difference between the two types of twins is based upon the difference or similarity in their DNA structures. Medically, identical twins are referred to as monozygotic twins and result when a single sperm cell fertilises a single ovum and the zygote divides at a later stage to form two foetuses. Dizygotic twins results when two sperm cells fertilise two ova in close succession, even on different mating sessions. The womb can be put under considerable pressure with the presence of twins and in fact many twins are born two to three weeks before babies that are not twins. Twins, whether identical or not, form only around 2% of the world population. DNA testing of twins to find out whether they are monozygotic or dizygotic is useful for a number of reasons. More about twins on Twin Zygosity testing.