Stage I lactogenesis (secretory initiation) takes place during the second half of pregnancy. The placenta supplies high levels of progesterone which inhibit further differentiation. In this stage, small amounts of milk can be secreted by week 16 gestation. By late pregnancy, some women can express colostrum.

What is secretory differentiation?

Secretory differentiation represents the stage of pregnancy when the mammary epithelial cells differentiate into lactocytes with the capacity to synthesize unique milk constituents such as lactose.

What triggers Lactogenesis?

In summary, interpretation of the data available from both animal and human studies is that the physiological trigger for lactogenesis is a fall in progesterone; however, maintained prolactin and cortisol are necessary for the trigger to be effective.