Swelling during pregnancy is normal due to Edema, which is swelling caused by the fluid collecting in your tissue. Edema causes your feet to retain fluids such as water. This is usually most prevalent in the 3rd trimester, and can be excessive when carrying multiples or twins. It is common to see your feet more swollen towards the end of the day or in the summer months.
You could also be experiencing swelling due to the pressure of the uterus on your pelvic veins. The excess pressure slows the return of blood from the legs and it begins to pool. As it begins to pool, it forces fluid into the tissue of your feet. Hormone changes can also play a role in swollen feet.
Call your doctor immediately if you begin to experience swelling in the face, eyes, or hands. These are symptoms of preeclampsia, which is a very serious condition. It is NOT normal if one leg is significantly more swollen than the other, and you should make an appointment immediately.