Many things in our lives lead to a change in bra size. Taking birth control pills, menopause, medications, a new exercise program, weight change, pregnancy and more can all affect the levels of hormones in your body. This can cause your breast size to change. If there is a huge change in size, you most likely need to get measured again. However, for small, temporary changes, simply adjust the straps on your bra or purchase a bra extender.