If you mean that she is approximately 13 weeks pregnant, it isn't going to happen. Paternity testing happens after the birth of the child.
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You have to wait until the child is born. If you you want to be sure you file a SAPCR and request a DNA test as soon as possible to establish that you are not/ are the father. Its best to do this early because you don't want build up a large amount of child support if you are the father.