When you are pregnant your body retains fluid.
This is called edema.
It causes swelling around the soft tissue of your feet.
While loosening the joints around your pelvis so the baby can get out the
pregnancy hormone relaxin loosens the joints in your feet and causes
them to spread, sometimes permanently.
Although the swelling
disappears after you deliver your baby many women find that their
feet never go back to their pre-pregnancy size. Once your feet start
to swell, you'll need to invest in some larger shoes.
National Geographic - In the Womb
Paralumun New Age Village