Yeh l know the feeling, been there.
My best revenge after hearing that very question was " yes actually we have been trying for 6 years, the doctor's have told us we can't have them, and thanks for reminding me once again".
It works like a treat, they shut up sooooo fast, and you don't have to worry about that damn question from them again. If their gonna make you feel like ****, make them feel like **** too.
Then you can walk away and have a good cry if you need to. Just make sure you slam the door in their face first.
I never got treated beter in my life after that, and you don't have to worry about passing it on to other family members and friends, cause the old grape vine starts working over time.
It also becomes such a relief to just get it out there, and you might actually find you get heaps of support you didn't expect, that help lift the burdon also.

cheers,
polygirl.