You can fly domestically until you are 36weeks and then you need a Drs note to say all is ok to fly! I have flown rather late in all my pregnancies and am flying at 31 and 32 weeks at least with this one..(the positive or negative or a pilot for a DH!! and all the family being interstate!)

There should be not troubles at all at 18weeks!!! Hosties are allown to fly in their 2nd trimester! If you want you could get you gp to write you a letter of confinment so you can prove you are only 18weeks, but surely you dont look that big??