Better not. I flew when I was 5-6 weeks, the plane was quite bumpy plus my trip was quite tough...in the end I miscarriaged at 10 weeks. Well it may not be because of the trip, but I have truly truly regretted about my decision of flying. I will never never do it again.
it's depend on your own health condition.
I feel well after 16 weeks, and go business trip in Asia, also I am planning a short trip to Japan in 20 week. Just wanna to have a relax holiday before my second child arrive.