Don't go up a size... I think they use vanity sizing on maternity bottoms. At Motherhood I have to buy a size smaller than my non-pregnant size and it's still too baggy. At Old Navy I have to try on everything because it seems like the sizes are not consistent.
I'm done with maternity jeans entirely though. I must be shaped weirdly or something because I've tried several different types and I cannot keep them up (even toward the end of pregnancy last time).
I got one of those belly bands from Motherhood Maternity and am wearing my normal jeans partially unzipped underneath.
I found some cute business maternity pants at a consignment shop and for whatever reason those will actually stay up. I like shopping those stores because the quality of the item is usually better than what I could afford brand new.