A survey is showing that three kids is the most stressful number to have, and after that it's easier. Since that is what my mother always told me, I will not complain to much about the lack of meaningful statistics given. She had cause to notice, having spread seven children out over nearly two decades. (I was the magic stress-reducing fourth child, too, so that may be significant.) I also noticed my stress level go down significantly when we went from three to four children, probably because I spent the whole time in between in labor.
This has not been enough to entice me to further exploits--not that an extra child would be so hard, it's the acquisition process I no longer have the stomach for (quite literally). Also, it was enough of a challenge finding a vehicle that would fit four kids and a wheelchair. Upgrading is not really an option.
But, I definitely believe I am less stressed than mothers with three, or two, or maybe even one child. There are so many things that just don't matter once you realize you don't have time or energy for them any more. I saw one mother write about how she found it so freeing to be reassured that "As long as your children are fed, clean, clothed, and loved, you are doing a good job." And I thought: clean? clothed? Why? Fed, yes. You can't get away without feeding them, although here economies of scale come in. Clean and clothed are definitely optional.
Or again, a lovely mother of one little girl posted on Facebook about the challenges of getting stains out of socks. And I thought: stains? socks? What are those?
Also, once you have four you have hit critical mass for playmates--chances are anybody can find somebody to play with at any given time.
There are stressful things, like looking at the grocery cart and bill and imagining what they will look like with four teenagers. I try not to think about that. It's too noisy around here to think, anyway. I'm getting very good about not thinking about things. Which is, I suspect, the key to a low-stress life.