Recently. the Q&A on this very website asked this question: "Are you considering having only one child? What factors go into your decision?"
My decision on that topic was made a long time ago. I always knew that parenthood was a life experience that I didn't want to miss. But not considering myself a "natural" at dealing with kids, and being of the opinion that it only requires one of them to make you a parent, I was pretty sure as early as high school that one child was all I wanted. (After all, every firstborn started out as an only child.) I also knew that getting a college degree and having a career were experiences I didn't want to miss either, and thought that working and parenting might be more manageable with just one child.
When my son was 19 months old, I reflected on the fact that when I was that age, my sister was born, and marveled at how my mom had managed two in diapers--not for me, thanks, especially since I was still working on my degree. When he was three, his dad and I had just finished college, I was starting my career, his dad was starting grad school, and the boy was off to preschool - money, time, and space were all too tight. When he was seven, his dad finished grad school and we moved again - and that might have been a good time to consider adding to the family, but I remained unsure about going back to the beginning when we had one child becoming more independent every day. When he was 12, we got a puppy, and for me, that was a condensed short-course refresher on life with a baby--and pretty well sealed the deal that I didn't want to go back there. There was also the fact that "Mr. Only Child" seemed pretty content with life as it stood. The fact that he had young parents may have been part of that, but he was always very good about keeping himself occupied and rarely seemed lonely, and the happiness of the child I already had was understandably important to me.
By the time my son was 18, his father was no longer my husband, and I was glad that there had only been one child to be affected by the divorce. At that point, I was also approaching 40 at a pretty fast clip, and my preference for only one child strengthened into a non-negotiable certainty.
However, that certainty really only applied to having more children at that point - specifically, starting back at the square one with a baby. I was open to the possibility - and likelihood - that if I married again, there would be children that came as part of the package, and that's exactly what happened. And the second time around, I've got two kids in my life, and I'm still a working mom.