If you start to notice that your legs are expanding in size (especially around the ankle area), then see a medical professional as soon as possible. There may be a serious underlying health problem.
Legs swell when fluid accumulates in the tissues. This is known as edema. Edema can result from sitting or standing in one position for long periods, eating a lot of salty food, PMS symptoms, and pregnancy. Edema can also be a side effect of some medications.
More serious causes of edema include congestive heart failure, kidney disease/damage, and cirrhosis (liver damage), weakness or damage to the veins in your legs, an insufficient lymphatic system, and severe, long-term protein deficiency from an unhealthy diet.