Yes, there are a few wacky bioethicist types who have tried to defend infanticide, and yes, we know active euthanasia of disabled children has occurred (you probably already know the history of denying Down syndrome babies food because it would be a “lifesaving procedure”). But few people agree with the former and, to their credit, all 50 states eventually outlawed the latter. 

The point is, some ideas don't have traction. Infanticide is one of them. It doesn't matter how logically inconsistent it appears to you. To you it's obvious that a late-term fetus and a newborn baby are identical. Yet this idea isn't accepted widely.  

Maybe you should think less about your moral framework and more about why women get abortions in order to figure out how to save human lives. No matter how logical your argument, it is useless if it doesn't persuade. By the way, I support the goal of saving human life. But 50 million women got abortions last year. Why?