Figure 1. Stages of postnatal mammary gland development.

After the developmental changes occurring during puberty, the adult mammary gland can undergo several rounds of development during pregnancy, lactation, involution, postpartum and postmenopausal. Highlighted are the known key events occurring in each stage. Excessive proliferation and side-branching occur during pregnancy, and differentiation occurs during lactation and finally during involution almost all cells die by apoptosis and a mammary gland is re-organized into a pre-pregnancy state.