If you intend to do the blood test from Ravgen or the DDC, I think I've heard 12 weeks, but look on their websites to check. I believe LadyBeeley is thinking of when a woman has an amnio or chorionic villii sampling; those do post some risk to the baby but are never done just for purposes of determining who is the father. The newer tests (that only test for paternity) can do it with a blood draw from the mother's arm and (I think) a swab from the potential fathers. It is well worth checking all of this information by contacting one or the other of the above labs. Do your homework and check the reputations of any other lab you see advertised. And be advised, it will cost a lot, with the prices I have heard ranging from $1,700 to 2,000.
Theyre risky while pregnant. Even courts will wait til the baby is born, so I couldn't really tell you.