During any given years kids go in and out of the system. So 672 K kids spent time in the FC system in 2019.
OK, but how many are currently in foster care right now?
You talk about them passing through it, but you shy away from the current number.
It's because it's only about 400,000, right? Which is far less than what the peak was back in the late 70's, if we are to believe your source.
The US grew from 205M people in 1970 to about 330M people today.
Yet, the foster care system only has about 400,000 kids in it right now.
So help me understand why you think more kids are in foster care now, post-Roe, than before Roe when the total number of kids has remained pretty stagnant since the early 80's?

