This happened to me, especially during the last trimester. I could not get it to totally stop becuase I was not able to be off my feet enough. But what I did do was drink a lot of water. Elevate my feet when possible. If it got really bad I would wear tent-hose which are basiclly special tight knee high socks that promote blood flow in your legs, Dr. can get these for you or you can get them at medical supply stores.
Make sure your drinking enough water, remain as active as possible, and elevate your feet above your heart when sitting.