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. It is clear from the identical and fraternal twins are born that identical twins, results from the same fertilised egg will have the same DNA whilst non-identical twins, resulting from the fertilization of two ova by two sperms, will not have the same DNA. 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. A twins DNA test will help to find out whether twins tested are monozygotic or dizygotic is useful for a number of reasons, amongst which, donor organ compatibility.