Hang out at your local movie theatre long enough and you will be amazed at how many preggos you see. I agree with @pregbelltrim The best places for candid hunting are places that have significant numbers of people passing through.

I would say year round, a mall is a good bet.

But for me, the best chance is amusement parks, zoos, aquarium, etc.  

Or if there is a temporary event in town, like a fair, carnival, circus, rodeo, etc., they all come out to attend so it's a very target-rich environment.

I think the discussion of those three locations is too American. In general in Europe and southern countries (no Arabs, map please), you can find pregnant women on a beach in the range of 5/6 for every 250 meters and in shopping centers like IKEA every 5 meters almost (it is barbaric), but it is not the usual thing to be a meeting point a supermarket or similar, it is more usual to see pregnant women in the park or on the street and even in public transport.

Conclusion, too much American survey that is limited to go from home to eat and eat at home.