Women may become pregnant within three to four months of missing a Depo Provera injection, according to WebMD. For some women, it may take up to two years before they are able to conceive.
WebMD explains that a Depo Provera injection needs to be received every 12 weeks to fully prevent pregnancy, though this type of birth control lasts for up to 14 weeks. Depo Provera is 99 percent effective at preventing pregnancy when the patient receives the injection on schedule, but this type of birth control does not protect against sexually transmitted infections. Common side effects of Depo Provera include irregular or stopped periods, headaches, weight gain, depression, dizziness and loss of bone density.