Alicia,
LLL's The Breasfeeding answer book,  c1991, pg,141  cites under the section,
breastproblems: constrictive clothing-"Encourage the mother to wear
comfortable clothing that does not bind the breast when she nurses and avoid
bras that are overly tight or whose inner band constricts the breast, such as
some underwire bras."
Hope this helps.
Elisa