I build wooden clocks as a hobby. The first step is to decide on a design. That design is certainly not a clock. The first step in producing a child could be a romantic evening. There are many steps to both, but the first step is far from the point at which you can reasonably say you have actually created anything. If you claim you believe what you do for religious reasons, I'll believe you. I won't agree with your belief, but I will believe you. To say you believe cell division defines a separate human life because of proven medical facts, just doesn't have the evidence to back it up.
That's just it .... that first step of cell division
starts the creation. That's what brings the creation (the human being) into existence. That first step IS the starting point of that human being's existence. How can you say that it isn't? Prior to those first cells dividing, there was nothing there, was there? Just the separate pieces (eggs/sperm) but no developing
being.
At the starting point, is that human being fully developed? No. Is that human being recognizable? No. Is that human being viable outside of the womb? No. But what it is,
is a human being. Just because it is in such an early stage of development that you don't even know what you're looking at does not make is any less of a human.
That's why pro-life people find abortion so abhorrent, why we have such a difficult time with that 'choice'. It's one thing if a woman believes she is aborting 'just a bunch of meaningless cells'; quite another if you look at from the pro-life pov.