Lactation is the production by mammals of milk, generally from the nipples. It is triggered by the presence of prolactin and Oxytocin, which cause cells in the breasts called alveoli. It is normal in females who have just given birth and will continue as long as they are nursing. In other situations, it often points to an underlying pathology, particularly when it occurs in males.  

In pregnant females, lactation is triggered by a combination of a number of hormones, particularly a sharp drop of progesterone and estrogen after giving birth.  

Lactation at Wikipedia

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