The biggest argument of the pro-abortion folks is that it can't be considered a human being yet because it has not departed its mother's womb. A specious argument, considering the knowledge we have of pre-natal development, but one that has held since 1973.
You'd be surprised how many pro-choice documents/books/material I read through that denied that. :/
The biggest argument I have heard is that the human is TOTALLY DEPENDENT on the mother for survival. I then ask if any of the people with this view are parents. One or two will admit to this. I then request that they leave cereal, milk, fresh nappies and some clean water in a room with their 2-month old, lock that door and come back in 4 days to see what they find. The point is immediate.
Unlike other animals, God made humans to have a long period of post-gestational learning to absorb before we can function independently. Thus, their point is totally specious. Baby horses don't have this deficit; they can run in a week. As such, human babies are totally dependent on their parents for years as anybody with children knows fully well.