Well, if you're dealing with the realm of fantasy and otherwise fiction, one could potentially get breast implants without the usual surgery (Star Trek transporters? Magic? Nanite-constructed implants --which is essentially the same as magic? The possibilities go on from there).
Perhaps a woman is abducted by aliens and to help nurture their species, she's implanted with hormone secreting breast implants that help her to lactate the special chemicals needed to nurse their young? I dunno.
