A considerable change occurs in mammary size and function in the transition from phase 2 to 3, concurrent with the joey beginning to leave the pouch. There is a shift in milk composition, a rapid increase in rate of growth of the gland and an increase in total milk secretion. Control of lactogenesis at the peripartum period apparently is not under the inhibitory control of progesterone as in eutherian mammals. Nevertheless, progesterone is still necessary for mammogenesis during pregnancy. It is unknown what prevents lactogenesis from occurring in late pregnancy in the species.
Prolactin seems to be the primary hormonal regulator of lactogenesis and lactation in marsupials.
Of course, this means that only the suckled gland becomes more responsive to prolactin. This is an example of local control of one gland independent of the others. Phase 3 mammary tissue histologically looks like fully lactating tissue of eutherian mammals, while phase 2 tissue looks more like the prepartum mammary tissue. Binding of prolactin is significantly further enhanced during the 3rd phase compared with the 2nd phase. In addition, concurrent with the transition from the 2nd to 3rd phases of lactation, there is a transient increase in prolactin concentration in the blood.