I have a wide diastasis too, although I don't have an umbilical hernia. I do have two hernias on each side of my belly. The hernias came from my first pg (twins), so they were there during my last pg, and they really didn't cause any problems. They were painful at times, especially if they got poked at all, or if I leaned over to pick something up or turned too quickly. Yesterday I had an u/s, and it was painful when the tech would press on those areas, but the good thing is that I didn't notice them during actual labor or delivery.
For the diastasis, I think you're definitely doing the right thing with the pilates. I'm actually a pilates instructor, so I know that is the only thing that I've found that "fixes" the problem. I notice now, though, that when I am lying down or reclining and have to sit up, I get that nice ridge popping up down the middle of my belly, no matter how much I contract my transverse abdominus! I guess there's no getting around that.
Is there some kind of abdominal support belt that you could wear to keep your hernia from popping out? I would think that your midwife would know best whether or not you should see an OB. Good luck!
Gayle, mom to Reagan and Ryan, 09/15/03, Cade, 12/08/05 and sweet Josiah, 05/23/10-07/16/10