Honestly, they didn't in the entire 42 weeks of pregnancy. That could be because I was 18 when I fell, 19 when I had her, and lived in low-waisted jeans so they only ever really had to fit my hips which didn't change. I got one of those belly belt things but it was never any good cos I just didn't need it. I pushed them down lower, ever so slightly, and that's all I needed to do.
(No, I didn't have a small belly - my baby was 10lbs 4oz... lol).