Humans tend to equate things acting, and reacting, like humans to be humans. A worm, touched, writhes around the ground to try and get away. Is it scared? Is it fearful? Is it in pain? When you pluck a leaf off a tree, the tree "feels" it. Should we avoid plucking fruit off a tree because it could cause the tree pain? What if the tree recoiled from having its fruit plucked? These are not humans, but when we imagine what we would perceive if it was happening to us, we hurt. It's empathy. It's a good thing we can identify with others pain. But it is a pain we create inside ourselves, because we personally identify with something as being human-like.
In this case, a well-developed fetus can look, act, and react, much like the worm, much like the human, and much like most things with the equivalent to a nervous system. The further along in development, the more human-like it becomes. Generally it's only when the fetus has a functioning and viable brain do we start considering it an actual life. Before that it's still just plans for a human. Two draft blueprints smushed together, given biological parts by the mom, with plans changed every step of the way because of of the environment, until you get a viable brain, a viable human. If the brain never develops, if the brain hasn't developed, no matter how it acts or reacts, it is not a person.That's the simple definition. We are our brains.
I feel for those who identify with fetuses. They ascribe to them person-hood, they ascribe things that do not belong, and so feel empathy for it. They feel for it as if it were really a person, and so feel the pain of that loss just as greatly as if it were true. Which, frankly, must suck. that's got to be gut-wrenching and feel like a terribly immoral thing to do. Which, if it were true, would be accurate. But it's not reality. It's not how things, how our biology, actually are. It is not your fault, and it is not your burden. It's okay to let go of that guilt and responsibility. You're good.