While I wasnt planning on becoming pregnant again (mainly b/c I have pcos and took me 4 yrs to fall pregnant with DD#2 ) I have been under a dietitian were I lost 17kgs on a low calorie diet (if I knew it was that easy to lose the weight I would have done it sooner) I'm also a diabetic type 2 which is controlling my sugars perfectly during pregnancy so much so that they took me off Metformin.
I cant recommed the low calorie diet more highly to lose weight and control diabeties.
Good luck.