Gigantomastia is a rare condition that is characterized by excessive breast growth that may occur spontaneously, during puberty or pregnancy, or while taking certain medications. To date, there is no universally accepted definition for gigantomastia; however, Dancey et al. (2007) state that a review of the medical literature suggests that definitions range from a D-cup bra size to breast enlargement requiring reduction of over 0.8 - 2 kg, which is equivalent to about 1.75 - 4.5 pounds. The exact cause of gigantomastia has not been determined. Nonetheless, the following theories have been proposed to explain gigantomastia: (1) end-organ hypersensitivity (a condition in which the breast tissue is more sensitive to hormones circulating in the body), (2) autoimmune issues, (3) high IGF-1 (insulin growth factor-1, a hormone involved in regulating bone growth) and (4) hyperprolactanemia (high levels of prolactin). Gigantomastia has been noted as a side effect of treatment with certain medications like D-pencillamine and in one case as an apparently hereditary condition. Symptoms of gigantomastic may include mastalgia (breast pain), ulceration/infection, posture problems, back pain and chronic traction injury to 4th/5th/6th intercostal nerves with resultant loss of nipple sensation. It is may also associated with decreased fetal growth, if the gigantomastia is present during pregnancy. Treatment is based on the person's symptoms and may include breast reduction, mastectomy with or without reconstruction, hormonal treatment, or a combination of treatments. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.