I'm basically doing that right now too.
I currently wear about a US size 10. However I have a number of clothes in 12's and 14's that are too big that I'm hanging on to at the moment. We're planning on TTC once I get to a healthy BMI so maybe at the beginning of 2012 (hopefully?). I'm guessing I'll be about an 8 then, maybe with a few 6's thrown in there.
I also plan on hiding my pregnancy for a bit because I've had a miscarriage before and don't want to have to tell anyone until I'm fairly far along (at work especially). So my plan is to just wear my old clothes until they don't fit anymore. Granted, the fit never works quite right but I'm ok with that.
I do have some plus-size maternity clothes and my hope is that I can either alter them myself or have someone do it because maternity clothes are insanely expensive here! Plus, some of the clothes are pretty much brand new so I'd like to be able to use them.
FWIW, last pregnancy I was able to wear my "fat pants" until I was maybe 10 weeks along? I gained 10lbs in the first trimester, though, so that's not the best example.
In my second pregnancy I actually lost weight and did pretty well in my non-pregnancy clothes for most of the first trimester.
Afterwards it's good to have some larger sizes around just for the in between stage. I remember buying some 16's at that time with the thought that I'd be out of the soon, sigh, I wish I would've kept that up at the time!
ETA: I wanted to also add that your body does change quite a lot after pregnancy. I carry a lot more weigh in my waist now than before so when pants don't fit me now sometimes it's because the waist is too tight (but the butt/thighs are not). Prepregnancy I always had way too much room in the waist so the pants that fit me then don't fit me quite as well now (my waist seems to finally be shrinking a bit now but pretty slowly!).