I flew Singapore airlines when I was over 30 weeks pregnant (can't remember exactly) had the medical cert to say it was a low risk preg and I was never asked for it. No one seemed to bat an eyelid TBH. They probably just felt sorry for me that I was so massive and travelling with a toddler in a confined space for 5 hours!

I'd also been on tiger airlines when I was about 25 weeks with no probs either.

Just check websites and have a certificate at the ready.