Stretchmarks are a perfectly natural reaction to periods of rapid growth or weight gain which cause collagen and elastin fibres to separate within the skin's central layer, the dermis.
Starting out as red, irregular lines, they eventually fade to white or silvery patterns instead. Keeping skin well moisturised may help it stay more supple, but is not proven to prevent damage occurring beneath the surface. Some topical treatments help disguise the appearance of stretchmarks for as long as they are used.
Otherwise, professional microdermabrasion treatments, Retin-A creams and long-pulsed laser systems such as Cooltouch and Smoothbeam may be of use, for which you should see a dermatologist.