I answered this question about a week ago so I have simply pasted my research response here for you, see the Baby Centre bit
I have had swollen legs, feet and ankles for a good few months and am 31+ weeks now. I am on my feet all day and so it's very hard to elevate them. I have found that being able to sleep on my left side again now that my rib pain is mildly better the swelling in my feet and ankles does come down overnight but is soon enough full of fluids again as the day goes on.
Have you got a dream genie pillow or are using another pillow between your legs, this might help by freeing up circulation.
Kankles look 'bleugh' so more recently I have been on the hunt for maxi dresses so at least for the best part of the summer before birth they are not on display & I just try and get feet up when I can, they need to be higher then your heart so that fluids can return and be filtered. It's really only a temporary message but makes me feet less swollen. Flip flops are also the only foot wear suitable now.
I Copied this from the Baby Centre Site
I hope this information helps................All the best X
Why are my ankles and feet so swollen?
What you're experiencing is oedema – that's when excess fluid collects in your tissue. It's normal to have a certain amount of swelling during pregnancy because you're retaining more water. Changes in your blood chemistry also cause some fluid to shift into your tissue.
In addition, your growing uterus puts pressure on your pelvic veins and your vena cava (the large vein on the right side of the body that carries blood from your lower limbs back to the heart). The pressure slows the return of blood from your legs, causing it to pool, which forces fluid from your veins into the tissues of your feet and ankles.
For this reason, oedema is most likely to trouble you during the third trimester. It may be particularly severe for women with excessive amniotic fluid or those carrying multiples. Oedema also tends to be worse at the end of the day and during the summer.
After you deliver your baby, the swelling will disappear fairly rapidly as your body eliminates the excess fluid. You may find yourself urinating frequently and sweating a lot in the first days after childbirth..
When should I be concerned about swelling?
A certain amount of oedema is normal in the ankles and feet during pregnancy. You may also have some mild swelling in your hands.
Call your midwife or doctor if you notice swelling in your face or puffiness around your eyes, more than slight swelling of your hands, or excessive or sudden swelling of your feet or ankles. This could be a sign of preeclampsia, a serious condition.
Also call your caregiver promptly if you notice that one leg is significantly more swollen than the other, especially if you have any pain or tenderness in your calf or thigh. This could signal a blood clot, another serious condition.
What can I do to minimize the puffiness?
You can help relieve the increased pressure on your veins by lying on your side. Since the vena cava is on the right side of your body, resting on your left side works best.
Here are some other tips:
•Put your feet up whenever possible. At work, it helps to keep a stool or pile of books under your desk.
•Don't cross your legs or ankles while sitting.
•Stretch your legs frequently while sitting: Stretch your leg out, heel first, and gently flex your foot to stretch your calf muscles. Rotate your ankles and wiggle your toes.
•Take regular breaks from sitting or standing. A short walk every so often will help keep your blood from pooling in your lower extremities.
•Wear comfortable shoes that stretch to accommodate any swelling in your feet.
•Don't wear socks or stockings that have tight bands around the ankles or calves.
•Try waist-high maternity support stockings. Put them on before you get out of bed in the morning so blood doesn't have a chance to pool around your ankles.
•Drink plenty of water. Surprisingly, this helps your body retain less water.
•Exercise regularly, especially by walking, swimming, or riding an exercise bike. Or try a water aerobics class – immersion in water may temporarily help reduce swelling, particularly if the water level is up near your shoulders.
•Eat well, and avoid junk food.
Try not to let the swelling get you down. The sight of your swollen ankles will probably add to your feeling of ungainliness, but oedema is a temporary condition that will pass soon after you give birth.