Maternity clothing is SO awful! I've bought and returned so much stuff, I can't tell you. I'm quite 'fashiony' (trying not to make myself sound awful here!) normally so was happy to invest in basically a whole new wardrobe of maternity stuff but to be honest, it's been impossible!
I've got a pair of Topshop Jamie jeans in maternity fit and although I love them and they look great, they're under the bump and fall down constantly. So that's not ideal. I got a couple more pairs of jeans from New Look which don't look as nice but are very comfy and much cheaper at £17.99 a pair.
ASOS is hit and miss - I've got some nice sundresses (which I'll hopefully finally be able to wear this week) and a couple of cami tops which are good with jeans. I also got a maternity maxi skirt which i wear with stretchy vests tucked in.
I found most of my regular clothes were out from about 14 weeks as I like high-waisted stuff and my rib cage has been massive from the beginning!
Also, H&M are the absolute worst for sizing. I bought a shift-style, loose fitting dress from their normal range a few weeks ago - I'm usually a 12, so bought a 16 in hopes that it'd fit over the bump. It's very baggy on the bump but tight on my hips! Go figure! 