So great to read your responses. Thank you!
ViviPru I have been trying to work out my answer to your really good question about why 2013 would be best. It seems to be a combination of good practical reasons and rather whimsical ones.
I have an unusual job, it is also sort of a fixed-term contract, so although someone could come in to cover me for mat-leave, I would not gain the full advantage of the good and limited-time job I have. It also might (only possibly) mean missing out on a raise, and with the expense of a baby I would like to have all the financial stability I could. I am also invested in the project and loath to hand it over. Waiting until Jan 2013 fits in really well.
And then you are right, I hung everything else on that. The earliest that would work for my job ideally would be October/November TTC. But December is the earliest for the school year thing (which my DP cares about), and then I will have to travel long-haul at Christmas, so it becomes January.
Other silly things - we have travel plans to a developing country at Easter (so not good if pregnant). I think DP also likes the idea of a time to get his head round it all. Although if I pushed he would go for it now and there is no way he is thinking about the 'baby maths' as Awayinamangercooper pointed out!
So 2013 is logical. It might also be foolish, I keep looking at that graph of my declining fertility and the other one about the increasing risks. Cheery stuff!
I never was good at decisions...