I'm in the same boat as SamiH. Age was the main factor for me.
I was on a diet that I'd paid a lot of money to do... but in my heart I wanted another baby. I don't want to be close to 35 when having my last so after much discussion with my DH, we decided that we would try for a few months and if nothing happened, I would continue on with my diet and we'd try again after I got to my goal weight.
Long story short, I fell pregnant the first month and I am happy because it now means I will be 33 almost 34 when this Bub arrives... and I can pick up where I left off after the bub with my diet and lose the baby weight too.