I must buy my clothes too big because almost all of my coats and my sweatshirts are still totally wearable.
I would never think of buying something size "small" because I do not consider myself small. I am a tough, sturdy, medium, oftentimes even a large.
The last batch of maternity clothes that were donated to me had a bunch of smalls in them and you know what? They fit me! Right now I have on a Gap zip up hoodie, which I thought fit me in real life, but I'm wearing it 4.5 months pregnant and it fits me just fine. There's growing room, even.
After I go back to normal, I may have to rethink my size, which may be a good thing.