<P> During the latter part of pregnancy, the woman's breasts enter into the Secretory Differentiation stage . This is when the breasts make colostrum (see below), a thick, sometimes yellowish fluid . At this stage, high levels of progesterone inhibit most milk production . It is not a medical concern if a pregnant woman leaks any colostrum before her baby's birth, nor is it an indication of future milk production . </P> <P> At birth, prolactin levels remain high, while the delivery of the placenta results in a sudden drop in progesterone, estrogen, and HPL levels . This abrupt withdrawal of progesterone in the presence of high prolactin levels stimulates the copious milk production of Secretory Activation . </P> <P> When the breast is stimulated, prolactin levels in the blood rise, peak in about 45 minutes, and return to the pre-breastfeeding state about three hours later . The release of prolactin triggers the cells in the alveoli to make milk . Prolactin also transfers to the breast milk . Some research indicates that prolactin in milk is greater at times of higher milk production, and lower when breasts are fuller, and that the highest levels tend to occur between 2 a.m. and 6 a.m. </P> <P> Other hormones--notably insulin, thyroxine, and cortisol--are also involved, but their roles are not yet well understood . Although biochemical markers indicate that Secretory Activation begins about 30--40 hours after birth, mothers do not typically begin feeling increased breast fullness (the sensation of milk "coming in the breast") until 50--73 hours (2--3 days) after birth . </P>

Short note on the process of milk let down