Ive flown three times alone while pregnant all around 5-6 hour fights... Let them know your pregnant before boarding they'll most likely let you on to sit before everyone else. Try to get an aisle seat for bathroom breaks I had one guy get so annoyed with me and another was actually super kind and switched with me ...
Also try to stand up and stretch when you can.
Dress comfortable. Bring tons of snacks possibly a small meal like a sandwich to snack on through out. I brought a book just because I was alone ahaha
I would bring snacks on my carry on bag crackers and such I have to eat small amounts every few hours or I feel sick in beginning of my pregnancy
20 hour flight, and at least 15 hours on planes!