cups I've flown "medium haul" so far (5-6 hrs) at 10 weeks, and have 2 long haul trips planned at what will be 20 weeks and 22 weeks. There is no reason not to fly apparently and lots of people do.
I usually favour window seats and just gettign cosy for the duration, but for these flights I have gone for aisle seats so I can get up regularlyand move around (to minimise the thrombosis risk) and also because I found I bloated up HORRIBLY on the flights I did at 10 weeks and was just massively uncomfortable sitting down.
BTW, my biggest concern was the backscatter radiation security machines (although they are apparently fine I am unconvinced). They use them loads in the US but the TSA or whatever it is called rules say you have a right to request a patdown instead (I have heard they get @rsey if you do but I really don't care), so if this bothers you, you might want to take a print out of the rules. I think in the UK though, they are used much more rarely, but if you are randomly selected you might not be allowed to fly if you refuse.