The technical term for dropping or sagging of the breasts is “breast ptosis” (try to use that one in everyday conversation!)
Rinker B, Veneracion M, Walsh CP. Breast Ptosis: Causes and Cure. Ann Plast Surg. 2010 Mar 29. [Epub ahead of print]
…History of breast-feeding, weight gain during pregnancy, and lack of participation in regular upper body exercise were not found to be significant risk factors for ptosis.