The legal definition of death is the absence of brain waves, ergo the presence of brain waves is the indication of life.
That is pretty much a definition I would go with.
I've actually had arguments with some FReepers over this, who try to make the case that my refusal to condemn birth control measures such as IUDs (which prevent implantation) is exactly the same as promoting abortion at 9 months. It isn't. A ball of 100 undifferentiated cells that has no subjective awareness of existence is not a three or four week old embryo with a detectable nervous system who is aware of and feels its environment.