As far as I know it doesn't matter. I think in the US women have smears done routinely during the first trimester (but I could be wrong). I've just had mine done almost mid-cycle and we are about to start TTC #2. I had been (stupidly) putting it off, it was a New Year resolution to get it done! Usually they prefer you to have it done mid-cycle so they may ask you to wait until next month anyway.