Yes, the mammary glands of the female undergo differentiation during pregnancy and starts producing milk towards the end of pregnancy by the process called lactation.

Mammogenesis is the process of growth and development of the mammary gland in preparation for milk production. This process begins when the mammary gland is exposed to estrogen at puberty and is completed during the third trimester of pregnancy. During pregnancy, the ductal systems undergo rapid proliferation and form alveolar structures within the branches to be used for milk production. After delivery, lactation which involves the secretion of milk by the luminal cells in the alveoli.