At 12 weeks, I am rapidly reaching the point where I can no longer wear my regular clothes. I have 3 pairs of jeans that I want to turn into maternity jeans. I found several tutorials online for adding bands or panels to jeans. It seems most of them are designed to be low-slung, cradling yet showing off a perfectly smooth bump. I don't have a smooth round belly, and I most likely won't! I have a lot of belly flab from previous pregnancies and it just hangs and looks awful. I mean, I really hate the way the hanging flab looks. How do I design a maternity panel that will support my bump and hide the flab, but not look awful? I don't want to spend my entire pregnancy hiding under tent-like blouses!
-Ren, "Space Madness", The Ren & Stimpy Show, 1991
Using a double layer of stretch jersey in whatever panel you choose will help smooth things out and give you more support. Perhaps adding an all round extension of 6" high or more will give you a smoother look too rather than an under belly design?
You don't have to have one trouser that fits from start to finish so experiment a bit. You could make a waistband for two pairs to get you through the next few months then make a much deepr waistband as your bump gets bigger and then go back to the earlier versions and upgrade them to bigger-bump-covering size.
Or you could start from scratch and sew some trousers. I've heard good reviews of Burda 8376.