This is a pretty big issue. It's definitely something you can work through together, but if you can't come to some sort of agreement, it could lead to resentment.
It's funny, I have a 5.5 year old, and after I had her, I said I was done. But at that time, my marriage was already pretty much over. I think being in love can change those feelings. I am at a place now where I can't wait to have one more...my clock is on overdrive. I never thought I would change my mind, but I really do want one more. His clock is just as fast or faster than mine at this point, which is a good thing.
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That being said, I told him that I knew I would only want one more. We had a very honest talk about that...that after, god-willing, we have a child, I really will be finished. I told him that I intended to get my tubes tied after delivery, and the factory would be forever closed. So he would have a stepdaughter and one child. I told him that he needed to be ABSOLUTELY sure that he could be happy with just one biological child, because I knew this time I would not be changing my mind. Because if he wanted more than one child, I would never want to rob him of that. I told him I would never want to hold him back from the life he wants. To which he replied that as much as he wants a child, if it came down to having me and NO children, or not having me at all, that he would never want to be without me. That was all I needed to hear.
Bottom line, it's important to have a very honest talk and be up front with each other about what you both want. Because if you aren't clear with each other now, there could be some serious bumps down the road. People can change their minds (obviously I'm proof of that), but you really can't count on that.