I really think that the biggest change is from having zero kids to having one. There were times when I didn't know what I was doing or if the exhaustion would ever end. I felt like I was driving in some strange land with no map ( Does that date me? perhaps, I should say GPS), and that's the difference- after your first you will always have a road map. Of course, the route you take with your second will be different from the first, but you know the general direction you are going. Trust yourself- once you've had one you are ready for more.