The advice differs for different stages of pregnancy and here are the tips:
For all stages of pregnancy it is recommended to have medical clearance for travel by your OBGYN or Midwife.
Stay hydrated (this means you should urinate each 1-2 hours while traveling) aim for around 100 fluid ounces the day before, of and after travel
Bring snacks to reduce nausea and stabilize blood sugar- crackers, nuts, low sugar granola bars
Walk and stretch once an hour while flying as long as the captain says it is safe
Bring preggy pops or something similar to help with nausea
Pack ginger candies to help with nausea
Mid to Late Stage of Pregnancy 5-7 months:
Stay hydrated (this means you should urinate each 1-2 hours while traveling ) aim for around 100 fluid ounces the day before, of and after travel
Eat a large meal before flying and bring snacks with protein to help stave off hunger
Wear compression hose (up to your knees) to reduce swelling of your feet- (I chose not to do this and suffered the consequences finding myself unable to walk more than 15 minutes while in Hawaii due to the extra humidity that caused swelling)
Walk and stretch once an hour while flying as long as the captain says it is safe
Bring a small lumbar pillow to relieve back pain or sciatica
If you get a severe headache during or after flying go to the nearest medical center to check your blood pressure. This happened to me in Hawaii and I took a Tylenol at night and went to the fire department to check my blood pressure. It was fine but I wanted to be safe, not sorry.
Late Stage of Pregnancy 8-9 months:
Not advisable to travel by airplane