Earlier quoted context omitted.
Interesting. From your perspective, what's the % of how often you have heard "biological" vs "birth" parents and has the rate changed any? I'm curious as to why the change of what I've heard (biological parents) and in the recent year (birth parents).
Most likely there has been no such change. You've just started paying attention to it for some reason. Looking at a couple of data sources for popularity of terms over time, "birth parent" has been consistently more used: https://books.google.com/ngrams/graph?content=Biological+par... https://trends.google.com/trends/explore?date=all&q=Biologic...
Also, I'd assume the child would later look for those people with their genetic material, their biological parents, those who provided the sperm & eggs. Not the birth parent, who birthed them. There's much less of a connection there as it's the person who simply carried them for a money transaction and is not part of the child genetically.