I always wanted two kids... except when I was 5, I wanted 3 kids all named Sarah... but that doesn't count. Anyway, after I had my first, it wasn't long before I started thinking about baby number two. After baby number two, I started thinking about getting a dog... and until I had both my kids, I didn't really want a dog that much. It was something my husband had talked about wanting and since the birth of my son, I've actually come to want a dog. As for the idea of pregnancy, childbirth, and having to do the newborn stage again and the idea of having more children around... no thank you! It was an amazing experience and it was interesting to do a second time, but I have ZERO desire to do it again. I'm looking forward to moving from the baby stage to the toddler stage and then having more freedom as my kids gain independence with age. So I guess my answer in short is I knew because:
1) I had ZERO desire to be pregnant and give birth again.
2) I don't want to do the baby stage again.
3) I started thinking about getting a dog instead of thinking about having another child.