We are thinking about a second kid now, and realized...we kind of screwed up. Women are really most fertile from their 20s to very early 30s (and also have the best chance of a healthy baby, etc...), it is a time when most people aren't even thinking about getting married yet these days, let alone having kids! When you finally figure out you are ready...well...it is much harder to have a kid.
So we really needed to think of this a few years ago. It might still be possible, but now it is an uphill battle and will be a much higher risk pregnancy to boot.