I had a recent conversation with Linda "Coach" Smith about this very issue (of whether to put hot or cold on a breast).  The conclusion was, if there is any edema (swelling) then only cold!  If the mother is having a hard time getting milk out, it is the presence of the swelling that is making milk flow difficult and any addition of hot can make the swelling worse, while cold reduces the swelling.                              

If there is no swelling, either will work (depends on what mom thinks feels better) and one "expert" says to put on warm compresses about 4 minutes before nursing to relax mom and help "open ducts up." 
Just my $0.02 on the subject!
Terriann Shell      Big Lake, AK

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