When the placenta separates, a hormone that held milk production back is
removed from mom's body (that's why some retained placenta doesn't let milk
come in).  Stimulation to the nipple, whether by baby or by hand or by
pump, signals the brain that somebody is requesting  and hormones better
make breast make milk (prolactin)  and eject it (oxytocin).  Colostrum,
then milk  left in  the alveoli  plus no stim to nipple tells the system
"oops no  milk needed this time" and production slows and stops as cells
involute because hormones are inhibited by lack of stim.  That's the quick
and easy version of an extremely complex system that we probably don't know
all about :-)  Sincerely, Pat in SNJ

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