There is normally no reason why you can't fly, unless of course you have developed a medical condition such as preeclampsia. Most airlines set a "cut off" time of around 32 weeks (this varies with each airline) after which they will not allow you to travel. This is because of the (albeit remote) risk of you going into labor on board the airplane. Also, long haul flights are extremely uncomfortable after about 24 weeks.
Ensure that you have plenty of leg-room and easy access to the toilet (an aisle seat is usually best) . Carry some bottled water with you in case you feel a little dry, and some easily digestible fiber / carbohydrate such as fruit or cereal bars. Avoid alcohol, as this will make you dehydrated, which coupled with the demands of your pregnancy, and slightly lower oxygen content in the cabin, may make you light-headed.
The link between increased risk of deep vein thrombosis (DVT) and long flights is not proven, bur the concern is serious enough for many airlines to provide exercise sheets and advice on how to reduce the risk while on board. Because of changes occurring to your blood composition in pregnancy, you may be more ar risk of DVT but you can take steps to reduce this. Wear comfortable, supportive shoes, keep hydrated with water or clear fluids, avoid alcohol, eat snacks, and try to move around every two hours or so. Follow the exercise advice onboard . Flex, extend and rotate each foot ten times and massage the calves of your legs every hour or so.