Blood tests may be used to exclude some men from being the fathers of some children. There are four main blood types: A, B, AB (A-B), and O. When two people have a child together, their blood types determine the blood type of their child. For example, if both parents have blood type A, their child could have either blood type A or O. It would be impossible for their child to have blood type B or AB. If a woman believes a man to be the father of her child but his blood type doesn't match as a possibility, he is excluded from being the father. Using a blood test to determine paternity is often far from accurate, however. If a woman believes a man to be the father of her child and his blood type matches as a possibility, he may be the child's father. Since millions of other people also have the same blood type as the suspected father, there is no way to know if he's the father for sure using a simple blood test. DNA (D-N-A) paternity testing is a more accurate way to establish paternity because each person's DNA, or genetic material, is unique, except for identical twins. A child receives half of his or her DNA from the biological mother, and the other half from the biological father. The results are more accurate with DNA paternity testing. If a man is shown to not be a child's father, he is 100 percent excluded as a possibility. If test results show a man to be a child's father, there is about a 99.8 (ninety-nine point eight) percent chance he is.