Hi,
When a mammogram finds that there is an asymmetric density, this usually means that an area of the breast is obscured and appears as a shadow. In this case,a diagnostic mammogram is needed to focus on the area so a diagnosis can be made.Very often the density will disappears upon compression indicating that it could have simply been tissue that folded over on itself.If the density remains, then further investigation will be recommended such as an Ultrasound which will provide more information.Although it is impossible to predict in advance what this very small density represents,try not to panic.The compression will give them a better picture.Very often,asymmetry density turns out to be nothing to worry about.
Wishing you all the best...