Answer: There are a number of things you can do. Request a seat in the middle of the plane over the wing for the smoothest ride, or a bulkhead seat for more legroom. In either case, reserve a seat on the aisle so you can get to the bathroom easily and get up to walk and stretch.
Sitting anywhere for long periods of time can make your feet and ankles swell and your legs cramp. Take off your shoes and elevate your legs by resting your feet on your carry-on luggage under the seat in front of you. If the seat next to you is empty, put your feet up on it. Since pregnancy makes you more susceptible to thrombosis (the formation of blood clots) and varicose veins, keep your blood circulating by strolling the aisle every hour and doing some simple stretches every half hour. Try this: When seated, extend your leg, heel first, and gently point and flex your foot to stretch your calf muscles. When seated, rotate your ankles and wiggle your toes. Get up once every hour or so to walk around, but make sure you sit down as soon as there's a whiff of air turbulence.
Always keep your seat belt fastened under your belly and low on your hips while seated. And drink plenty of fluids, especially water, to stave off the dehydrating effects of dry cabin air. Caffeine can act as a mild diuretic, so it's best to avoid coffee, tea, and certain soft drinks before and during the flight. And beware of consuming gas-producing meals or drinks before takeoff. The trapped gas from foods such as cabbage and beans expands at higher altitude, making for an uncomfortable trip.
Wear whatever makes you comfortable. Beyond that, it's best to wear a dress or separates, such as a skirt or pants with a top (rather than a jumpsuit or overalls), to accommodate frequent trips to the bathroom. Stick to outfits with few or no fasteners — that helps speed up your pit stops. Layer your clothing to cope with sudden temperature changes. Since your feet will probably swell, wear comfortable shoes with expandable elastic panels, laces, or adjustable straps. Bring a thick pair of socks or some comfy slippers to wear around the cabin if you take your shoes off. If you're prone to varicose veins or swelling, wear maternity support panty hose, which relieve swelling and aching in your legs by strategically compressing your veins to keep blood flowing.