Immediately following parturition, the female begins to lactate and enters a period of reproductive repair called the puerperium.  For a period of time these two processes overlap.  During the puerperium uterine involution and return of ovarian function occurs.  Involution is the reduction in size and “remodeling” of the endometrium so that the uterus can initiate and sustain another pregnancy.
Mammary gland development is initiated prenatally in the female fetus and continues through puberty and pregnancy.  The anatomy and distribution of mammary glands is diverse among mammals.  Secretion of milk does not begin until shortly before parturition (hours).  Lactation provides the neonate with the opportunity to nurse and be nourished with minimal expenditure of energy.  It also provides immunoprotection for the neonate because initial mammary secretions called colostrum contain antibodies that provide passive immunity.  Lactation continues until the neonate is weaned.  After weaning, the mammary glands undergo involution and return to a non-secretory state.