It’s a false argument. The unborn child has rights too like any human being. Libertarians who support abortion are shallow and can’t think it through.
I’ve heard that libertarianism is all about the individual. That’s also why real libertarianism, so I’ve heard, doesn’t believe in national borders, and therefore, people are free to come and go as they please. IOW, one would expect them to be pro-amnesty, anti-wall, etc.