I'm done with maternity sewing now (at 36 weeks I'm more about nursing wear now) but I did a lot of modifying non-maternity sewing patterns
into maternity clothes. Mostly I sewed lots of stretchy knit or shirred skirts, but I also made some dresses and knit tops. Even though I have gained 40 pounds and got quite the belly I've also found that RTW non-maternity knit skirts have worked throughout my pregnancy.
One common tutorial I've seen is how to convert non-maternity pants or skirts by cutting out the front area and sewing in a panel of ribbing. I got bigger in the hips as well, so that wasn't really an option for me. But you could try it:
Kwik Sew sells a pattern with instructions and a panel pattern for doing this, Kwik Sew 3324.
And here's a free tutorial
for doing the same.