Unwanted Comments from others...

Jessica
I work in a school and often I get the same sarky comments "are you sure you don't have twins in there" or "wow I thought you was ready to give birth" or "I heard you was having triplets!" etc... And loads more on top of that. It's really getting to me, I'm just over 20 weeks and have quite a big bump but why these people feel the need to make comments like this baffles me?! I don't know these people well enough to have a joke with even. 
Having a bad pregnancy as it is with bad SPD as I did with my first (ended up on crutches in the end) any way my question is...
 what responses can I give to these people who think they can say what they want to me about my appearance and however they want to..? Please help x