How Depo Provera Works

• Depo Provera is a type of birth control administered via an injection in either the upper arm or hip every three months. The injection contains synthetic progesterone. Progesterone primarily prevents pregnancy by stopping ovulation. However, it also thickens the mucus in the cervix so that sperm cannot reach any eggs that may be released. In addition, it thins the uterus so that a fertilized egg cannot implant itself in the uterus.
  
  

Duration of Effectiveness

• The Depo Provera shot is effective immediately if it is given right after childbirth or if it is given during a menstrual period. However, if the shot is given at a time other than those, it may take up to a week for it become effective, in which case another type of birth control should be used the week following the injection. The shot begins to lose its effectiveness by about 13 weeks after the injection. At that time, women using this type of birth control should use another type of birth control until they are able to get another injection.
  
  

Effectiveness

• According to Babycenter.com, the Depo Provera shot is 99.7 percent effective in preventing pregnancy, meaning it's estimated that about three out of every 1,000 women receiving the shot may get pregnant. It is recommended that the injection site not be rubbed for a few hours following the shot because rubbing may reduce the shot's effectiveness. Women getting the Depo Provera shot should get another shot every 11 to 13 weeks to continue preventing pregnancy.