Who defines 'medical necessity'?

Lounging on the beach one afternoon, my wife suggested that health insurers be required to reimburse women for breast reshaping after childbirth. After all, we treat delivery as a medical procedure and recommend breast-feeding for the health of infants. Why shouldn't fixing a "side effect" of this necessary biological activity—sagging breasts—also be deserving of insurance coverage? Reconstructive surgery after other medical procedures is reimbursed. In my gut, something tells me breast reshaping isn't really healthcare. But why? We already reimburse for a broad variety of cosmetic procedures, usually to fix a congenital deformity, an injury, or the effects of a disease.