When is a fetus an independent organism?

One of the arguments that abortion proponents use to justify the practice is that a fetus is not an independent entity, but merely part of he mother's body. A case in Britain is challenging that argument as a baby born at 22 weeks has survived, and is doing well enough to go home. Check out the pictures in the article to get an idea of just how small this child was when born - smaller than a ballpoint pen.