A milk letdown is when milk, upon being asked for, travels down from the alveoli at the back of the breast to the ducts near the nipple, ready for sendoff. Of course, letdown happens when baby is nursing (several times each nursing session). But it also happens with other stimuli.
These are my body's favourite things to letdown for:
Riley's smell (including his dirty laundry)
Rileys snuffling, grunting, or snorting
Skin to skin contact with Riley
opening the freezer
hearing ice crack in a glass
cold air anywhere near my chest
Ayden crying in earnest
random letdowns if Riley has not fed in awhile
other women have told me of these letdown triggers:
the sound of their breast pump
the cat's meow (seriously)
Ah, milk. Breastmilk is so nutritious a two year old can get 50% of its nutritional needs met by nursing. Scientists are now researching the potential in stem cells found in breastmilk. A baby exposed to germs will have antibodies specific to those germs made in its mother's milk, even if the mom herself was not exposed to the same germs. Hooray for nature!