Mammograms are not contraindicated during breastfeeding.  They are,
apparently, harder to read, but if they are being used to define a specific
lesion, this is not a big problem.  For screening, they become less useful.
But there is no evidence anyhow that screening mammograms save lives if they
are done in women under 50 years old.

Jack Newman, MD, FRCPC (waiting for an avalanche of rebuttal)