We can know three things about the unborn that tell us when human life begins. We know the unborn are alive, they are human, and they are whole organisms.
First, The unborn take in nutrients and grow via cellular reproduction.
Second, The unborn are offspring of human parents, and have human DNA.
And Finally, The unborn are human organisms like you or me whereas sperm, egg, and body cells are merely human tissue and do not have any rights.