Proliferating, low power. During pregnancy there is proliferation of the duct system, with the eventual formation of alveoli. A small amount of ductal growth, under hormonal influence, occurs during each menstrual cycle. But proliferation does not become marked until pregnancy. First the ducts and then the secretory elements (alveoli) are formed. The dense fibrous c.t. forms septae between the lobules and the amount of adipose tissue is reduced.