Hate to break it to you, but it's no myth that moms-to-be sometimes go up as much as a full size during pregnancy.
Don’t freak out just yet, though; the good news is, not all women experience this particular side effect, so there’s still a chance you won’t either. The size change is usually due to how much fluid you retain and how much you swell while pregnant. Also, if you've gained a lot of weight during your pregnancy, some of it might be going to your feet (hey, no place is safe!).
But calm yourself with this thought: After you give birth, the swelling will likely go down and your feet will shrink to their normal size—especially once you get back to your prebaby weight. If not, it's just an excuse to buy new shoes.