Upgrading your seat will be the biggest benefit. If you're on BA econ+ has recliner seats (with leg lifts) on their international flights, and business has an actual BED. I'm afraid I'm not as familiar with other liners and their seating options, so look into yours. Even if they're just normal chairs, the added room to stretch out will help with lymph return (what causes swelling) and bloodflow... not to mention allowing tray tables to actually come DOWN instead of sitting on your belly at an angle which would pour food down the chair in front.
Talk with your doctor (have them consult an OB if needs must, or recommend one to you to phone) before taking anything. Also be aware that prenatal vitamins ca *cause* swelling (and migraines, and nausea/vomitting) in many women. If such is the case, just take kid's vitamins throughout the day... don't torture yourself with being ill.