I'd just go with a polite "no, not yet" and then change the subject. I usually find that asking someone else about their children will usually divert them from wanting to question you about your intentions to start a family. Until you get to know people, it's usually easier to give the impression that you're not ready for children yet (or your DH isn't)... it's only when I know people better that I let them know the truth about our infertility and assisted conception.

BW