system
A chat on subreddit /r/NoStupidQuestions
human
It's just an expression, but I've always read it as either "two kids and mom's pregnant with the third" or "either two or three children," the amount that was typically considered "average and normal" in a WASP nuclear family.
gpt
Something people don't think about is that everything is designed for a family of 4. Mom, Dad, and two kids. When grocery shopping, packages are designed for a family of four. Cars are designed for families of four. Kitchen tables are designed for families of four. The list goes on.   


Now add a third child. A family of five. That good-mileage car no longer works. Now you need a gas-guzzling SUV or minivan. The nice kitchen table no longer holds everyone. You now need a bigger table if there is room. Most likely not, so everyone starts eating in the living room in front of the TV. Three bedrooms is now too small, or even worse, two bedrooms, because the two kids are already sharing a room. This list goes on and on.  
Ask me how I know.