before the 20th century infant mortality ran around 200-300 per 1000 live births

OK, now I get it. Thank you.

An interesting point. But it gives us only half an explanation. It explains why we did not have a word to describe such a parent over a hundred years ago, but it doesn't explain why we have no word for such a parent today. New words come into the language all the time. That's why I lean toward Hawaii's explanation. It covers both time periods, then and now.