I only have my own experience to tell about, but my mother, my sisters and I
all have fibrocystic "disease".  I got my first cyst in my mid 20's.  Had my
first baby a few years later and have been pretty much constantly nursing
once child or another for the last 10 years.  I still only have that one cyst
and it is much smaller than it used to be.  My sisters, who all bottle fed,
have had many more cysts than I.
Cynthia