I've found that sizing up just leads to 'tent-like' appearance.
I'm not sure why maternity designers think women need extra width, LENGTH!! length is what pregnant women need.
I'm 5'1 and didn't always have this problem. With my first 3 pregnancies, it wasn't an issue but I'm assuming it must be newer clothes that it's an issue. My last two pregnancies my clothes have hardly lasted throughout the whole pregnancy. In the beginning, I need smalls... midway through a medium and by the end a large. I can't say I'm any larger than the average pregnant mama but that's a pretty wide spectrum of sizes. I've actually had better luck (until the very end) with using regular non-mat clothes, the longer shirts being in style this time around has been awesome. Of course, I'm now 39+ weeks... not a whole lot that fits is comfortable haha.