Hello,
     The ectopic breast tissue that goes from the breast into the axilla
(armpit) is called the tail of spence.  It is normal breast tissue that is
probably functioning and contributing to the lactation.  Most women have this
but how much glandular tissue is in this region will then contribute to its
growth during pregnancy and lactation.  It is not a mass or abnormality.  It
does get incorporated into a mammogram and should be examined as part of a
breast exam.  Many women though will not notice it because of the shape of
their breasts or that there is less glandular tissue in this area and less
growth ect.  It will involute post lactation and become less noticable or even
disappear.

Dr. Cynthia S. Marske DO
Internal Medicine