Oh, I don't mind it if friends do it (most of them ask or quickly step back and say "sorry! you must hate that!!), but this random woman came up to me in this shop, grabbed my belly and started rubbing and jiggling it saying "oooh, I just loooved being pregnant! you must toooo". Not only was I just plain shocked, but my m/s is still around and she's very lucky she didn't get thrown up on with all the jiggling. That might have sent the right message. Heehee.

Surely if you just tell your MIL that you're not comfortable with it she'll understand. Or maybe you can just make sure you have one or both of your hands resting on your belly to block her when she goes for the touch.