Strictly speaking, life does not have a beginning. It is a property of the cells which are produced by the mother's and father's body. As long as those cells are maintained in favorable conditions, they will stay alive for a few hours, giving them time to encounter each other and fuse to become a new zygote, which is also alive. About half of the zygotes survive to implant and become embryos. When the property of life is lost, it does not return.
To use the presence of brain waves as a point after which abortion should not be allowed actually leaves open a window of a few weeks (4? I'm not sure) in which to abort.