From Wikipedia
Breast augmentation is a plastic surgery procedure by which either a breast implant or a fat-graft implant is emplaced to the thorax to increase the size of the breasts in order to correct congenital defects of the breast and of the chest wall. Consequently, after the breast-enlargement surgery, the symmetrical breast hemisphere is of proportionate size, has a smooth contour, and is anatomically consistent with the woman's body.