It's usually OK to travel up to the 35th week of pregnancy but airlines may differ in this ruling.
This link gives you further advice.... http://www.flighthealth.org/pregnancy.htm
Generally UK airlines accept you for travel freely up to a certain stage around 20 weeks (varies by airline), after that you will need a letter from your doctor saying how far you are along, your due date, and that you are fit to fly and without complcations. That will then be Ok up to about 32 weeks.
If you are below 2o weeks and especially big for your time, you may be as well to carry the letter anyway as proof of your due date.
Yes I would not recommend flying within the first 12 weeks and then insurance companies do not like to cover you for your last 10 weeks or so. hope this helps
Hi was going to post you the same link as you have in your first response which relates to flying in the first 12 weeks and there facts relating to problems which could occur. I. I flew when I was 30 weeks pregnant and had to get letter from my health visitor and seek seperate insurance as my holiday company advise me I would not be covered for any hospital treatment or early arrivals ect so I got extra to be on the safe side but that may have been required by the airline.