It is very common for your feet to grow a whole shoe size after you have a child. During pregnancy many women experience edema, swollen feet, which can make fitting into a regular shoe quite difficult. However, once the child is born and the swelling goes down, many women notice that their feet have grown a whole shoe size.
The cause for this comes from the hormone relaxin which the body releases while you are pregnant in preparation for child birth. The hormones main purpose is to relax the ligaments in the pelvis but it tends to relax the ligaments throughout the whole body. The ligaments in your feet will be looser and allow the bones in your foot to spread, therefore making your feet bigger.
Some women go back to being the same size while others will go up a half or whole size forever. At least you have a great excuse to do some shoe shopping!