From where do you think our science and biology derive definitions? When is the exact moment of death? A stopped heart? Low brain activity? (Oops, perhaps we shouldn't include that as you'd be considered dead.)
However, if you insist on going the scientific route, no problem. A human being is described as an organism. An organism is described as something able to carry on the processes of life. Self-contained, if you will. That does not mean another organism breathes for it. That does not mean it requires the organs and bodily functions of another organism in order to carry on the processes of life.
That said, it is we, society, who determine what constitutes an organism. It is we, society, who determines a human being has to qualify as an organism before it can be considered a human being. That is our "SCIENCE of BIOLOGY".
The bottom line is by classifying a fertilized cell or fetus as a human being it automatically results in denying women one of the most basic and fundamental rights and that's the right to their body. It is a vile and insidious goal to devalue and virtually enslave half the human race and, thankfully, virtually every free nation has people in power who fully understand and reject that out of hand.