Its effects last for 12 weeks, then another injection must be given. After 13.5 weeks, the shot is no longer considered effective. A woman MUST repeat the injection every 12 weeks to maintain protection from pregnancy. Steps to follow a complete gyn exam with a PAP smear within 6 months prior to starting the shot. Depo-Provera tends to make a woman's periods less regular, and spotting between periods is fairly common.
Some women stop having periods completely.
If your pattern of bleeding concerns you, return to your health care provider to get a blood test for anemia, to rule out the possibility of pregnancy, or to rule out the possibility of infection. You and your partner should use condoms to prevent spreading these diseases.