The point is, nobody knows when human life starts. The logical response to this reality is, Don't kill anything that might be a human life.
Here's an analogy that I like. Imagine a forest that is used to grain artillery weapons. Monday morning rolls around and a day of practice is about to start. The CO is informed that a large group of "hippies" camped out in the forest the preceding weekend. He asks the First Sergeant if all the Yoots are out of there. The First Sergeant responds, "Probably." THis is not satisfactory to the CO. Unless he knows that nobody remains in the forest, he will not start shooting. The First Sergeant sends a platoon out to see if they see any hippies in the woods. They come back an hour later. The First Sergeant tells the CO, "We are pretty sure there is no one out there." Still, the CO will not commence firing.
In fact, he will not start the operation until he is absolutely certain that no living person is in those woods. No matter what it takes.
You don't kill a living thing unless you are absolutely certain that it is not a person. Which is not possible with respect to a developing human, regardless of the stage.