It is not your imagination, nor is the swelling after pregnancy the reason for shoe size changes.  In fact, the same hormone changes that occur to allow birth of a child (loosening of the ligaments in the pelvis) can also cause lengthening of the feet usually upto 1 shoe size.  As the ligaments of the feet relax from the hormone, the foot will flatten out and lengthen slightly.   This increased length causes a change in shoe size that usually doesn't return to previous size.  Sorry, but this change is usually permanent. 

Good news is, however, that the shoe size doesn't continue to change every pregnancy, so additional births usually don't cause additional size change.

Brandt R Gibson, DPM
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Podiatrist, Neuropathy Doctor, Father of 11 and Founder of Mountain West Foot & Ankle Institute