Like Ivana, once I got to the point where I was showing enough for people to be asking I used the months before my due date.

So, I'm due March 19, on Dec 19 I said I was 6 months, Jan 19, 7 months, etc. Before that nobody asked 'cause they either couldn't tell or weren't sure, so if it came up and I told them I'd go by weeks (plus it seemed so far away at that stage!).

Now I tell them I have 'less than 2 months to go, but if she wants to come anytime after my birthday in the middle of this month she's welcome!'