OP - have you ever tried to discuss with your MIL the things that upset you first time round - criticising choice of nappies, poking your boob etc? If not, perhaps you could get your dh to tell his mum that though he's sure she did it with the best will in the world, her criticisms etc really upset you last time, and that's what you want to avoid this time.
Then, as others have suggested, let her come and visit during the first week - when your aunt will be on hand to give you moral support - and if she starts getting critical again, either take yourself and the baby off upstairs for a rest, or say, firmly but politely, 'this is how I am going to care for this baby, and your criticism is hurtful not helpful - please cease and desist.' Plus, of course, this time your first dc will be there to demand some of her grandmother's attention, which may take a bit of the heat off you.
Good luck for the birth, and I hope it all goes peacefully afterwards for you.
NanaNina - for what it's worth, you come over as very biased on the subject of mothers in law. I do get the impression that you think all mothers-in-law are hard-done-by, misunderstood and caring individuals and that all 'you young mums' have decided on a whim to hate these perfectly reasonable individuals. You don't seem capable of looking at individual cases and seeing how utterly unreasonable and toxic some mothers-in-law actually are. And please note, I do get on very well with my own mother-in-law, always have, and always look forward to her visits - so I have NO axe to grind here.