My longtime friend E, who lives in LA, is joyously awaiting the arrival of a new baby this fall. As luck would have it (and completely unbeknownst to me till she emailed on the weekend to advise), she has ordered / is wearing the Isabella Oliver maternity line.
Here's what she has to say:
The Isabella Oliver stuff I have bought has been sized 0,1,2 etc. For pregnancy they say to buy your pre-preg size and that has worked for me, so I would guess that they are pretty true to size on the non-maternity wear too. The skirt is definitely meant to be a tight fit, it's cut that way. I have found the staff to be extremely responsive by email so I'm sure if you sent them an email and included some measurements they would happily recommend the right size.
She sent me some photos of herself looking fantastic - but I fear it would not be fair to post them without her approval... E: Let us know if I can put them up on the blog.