It's a philosophical dilemma that demands rigour. When does a human person begin? If a few cells cannot be a person, well, adults are only bigger clumps of cells. What is the threshold? If personhood begins when the cells are independently viable, well, most infants are not independently viable and some elderly aren't either. What is the threshold? By contrast: The moment when it all begins and the DNA is first unique, and the potentiality for an adult is present is very simple and philosophically elegant. If personhood comes later, such as implantation, birth, 7th birthday, etc. it is difficult to give a basis in the philosophical sense. Many philosophers have struggled with this and it's no simple thing.