Doctor Oz did a segment of “Am I Normal” and was asked if it is normal for your shoe size to increase by one size after having a baby. 
Dr Oz said that it is normal for your feet to get bigger after pregnancy. Hormones called Relaxin are secreted to help the baby grow and to help the baby come out. Relaxin also relaxes your ligaments and so your feet often grown by 1/2 a size to 1 size and your feet do not go back to being their smaller size. Also, when you are pregnant you weigh more, and the weight gain during pregnancy can relax and flatten your arches so that you get bigger and flatter feet. Oh what we women go through for our babies!
Debbie Keds says
Many women don’t realize this fact until nature sneaks in this naughty little practical joke on the avid shoe lover. Ahhhhh. My perfect 7 1/2 to 8 medium feet became long and narrow. Now most shoes that fit me range between 8 1/2 narrow to 9 extra narrow. But I’m really happy because my 5’4\ petite sized body looks even better in a pair of 4\ classic narrow stilettos in the larger sizes.