Questions about flying when pregnant!

I am currently nearly 7 weeks pregnant, we booked a carribean Holliday just before we found out we were expecting.

flying guidelines just changed to flying right up until due date however Thompson require a fit to fly cirtificate over 28 weeks- I will be 29 going and 30 coming back my query is that the Docters adds to non existent week onto pregnancy therefore I technically am only 28 weeks!.

clearly if there is anything wrong at the time we won't go as baby is important however we need to make a choice about what do now, has anyone experienced airport problems with previous pregnancies?

Thanks in advance x