As others have said, declare you pregnancy to the insurer before you buy your policy and purchase the best possible cover - even if this means scrimping elsewhere in the holiday budget. Priority number 2 for money would be the most roomy seat you can afford - business class if possible, then premium economy, or whatever gives you most legroom and definitely an aisle seat. Your risk of DVT is massively increased beyond your non-pregnant state and you do not want to take any chances. Follow all the guidelines for moving about, hydration etc to the letter.
Ask your doctor or midwife if you can get higher-grade compression socks - the ordinary flight socks may not be enough. If they don't recommend anything specific, ask your pharmacist and make sure oh are measured and correctly fitted for them.
Obviously everyone is different but Personally I would only risk long haul to visit family, not just a general holiday, so that if something did happen necessitating a longer stay, then you have something of a support network nearby. If you did develop DVT or PE for example, it could potentially be weeks or months before you were cleared for the flight home. Not to mention any risks of premature labour, etc. So think very carefully before you book.
And check, check, check again your insurance policy, including coverage while abroad and terms for cancelling the holiday in advance if needs be.