What do you say to that?

I have been TTC for 3 cycles now and most of my co-workers know that we are trying for baby #2. So they are constantly asking if I am pregnant. Which currently I am not, just finished cycle #3 and AF due in 13 days. My question is God willing when/if I finally do get pregnant and they ask me before 12 weeks, what is a polite way to NOT tell them? I am more than willing to spill the beans but not at 4 weeks or even 10 weeks. I don't want to flatout lie, but feeling like thats the only option.