This skin patch is worn on the lower abdomen, buttocks, or upper body (but not on the breasts). This method is prescribed by a doctor. It releases hormones progestin and estrogen into the bloodstream. You put on a new patch once a week for three weeks. During the fourth week, you do not wear a patch, so you can have a menstrual period. Typical use failure rate: 9%, but may be higher in women who weigh more than 198 pounds.
(Info from CDC – http://www.cdc.gov/reproductivehealth/unintendedpregnancy/contraception.htm)
PLEASE read important warning information about the Patch – here