There are many websites and products online that claim to have ways to enlarge a female's breasts. One common technique that they use is simply a vacuum pump. There's no denying that these temporarily cause swelling, but what's less clear cut is if they actually enlarge the breast permanently, and if so, if they do anything more than merely stretch the skin out (which would create saggier breasts, which are usually not what the user would desire).
There seems to be a great deal of anecdotes floating around, but these are problematic as there's no way to know if they're legitimate (eg, as opposed to fake comments created by the manufacturers of these pumps) and they're certainly not controlled experiments.
