During pregnancy your circulation is slower and the blood volume increases. This results in blood and fluids piling up in the lower extremity. At times it can be due to excess sodium. Start with elevating your legs, do not sit in one place for long or stand in one position for long. Then if it does not help you can look at diet changes.

you can keep your legs at a height while sitting. keep 1-2 pillows (as comfortable) below your knees/legs. it will be of great help. also always keep some stool/support below your feet whenever you sit.

that is due water retention in our body ..plz reduce salt intake