Saw some survey results recently: apparently three is the most stressful number of children.
I'd agree with the comment in the article that most of the stress is self-imposed and the result of excessive perfectionism, too. That's definitely a big part of the problem around here, anyway.
My experience was that the transition between one and two kids was harder than the one from two to three, though. By the latter point, you already know how to juggle, and are just (ha!) adding a ball. Maybe that's why the stress levels for moms of four or more went down in the survey...they could just have the juggling thing down cold.
But I don't plan to find out personally: I think that one additional ball would probably send me right over the edge! I'll quit while I'm behind...