Breastfeeding women can have mammograms.  They are a little more
difficult to read, but if there is a previous one for comparison, there
is no problem.  Even if not, the mammogram can be useful.  It is absurd
to suggest weaning before having the mammogram, because how long does a
woman need to have stopped breastfeeding before she is considered to be
no longer lactating? If there is some urgency to make a diagnosis of a
lump, mammogram is not the way to go, and other methods can be more
revealing (ultrasound, needle biopsy, watchful waiting etc).  Lawrence's
book mentions that mammograms are of no concern during breastfeeding.

Jack Newman, MD, FRCPC