During the first trimester of pregnancy the mammary gland undergoes a major hormonal stimulus and this is why many women experience breast pain at this early stage of pregnancy.
This is because internally the mammary epithelium (which will produce breast milk) is developing and proliferating.

During breastfeeding, the glands present a great mammographic density due to this process, and this increased density affects the mammography accuracy, making the examination of little use during breastfeeding

Therefore mammography during pregnancy or while breastfeeding should only be performed if there is a suspicion of cancer.