You could call that the "hidden variable" theory of life. There's a spark of life that begins somewhere, but we don't know it exactly so let's err on the side of caution. That could very well be true, but it's slightly different from my argument. I contend that there is no objective spark of life. We can call whatever we like "alive." It's just a word, one that only has meaning to the extent that we find it useful. Hence if we pick an arbitrary moment such as the start of the 3rd trimester as the beginning of life, it's not a priori correct or incorrect. It's only right or wrong to the extent that it supports or conflicts with principles that we hold more important. (E.g. I personally would hold the bodily autonomy of the pregnant person to be more important than assigning the right to life to a fetus pre brain-activity.)