Once a Depo-Provera shot is missed, there is a possibility of pregnancy when having unprotected sex, according to the Indiana University Bloomington Health Center. It's necessary to have a Depo-Provera shot every 13 weeks for it to remain a viable method of birth control. If a woman misses a shot or receives it late, it is important to use a condom during each sexual encounter to prevent pregnancy.
According to the Indiana University Bloomington Health Center, a Depo-Provera shot is up to 99.7 percent effective; however, the medication must be injected into the buttocks every three months to maintain efficacy. The shot is effective as soon as it is received. Containing progestin, the Depo-Provera shot works by preventing ovulation, thickening the cervical mucus to prevent sperm from entering the uterus and changing the lining of the uterus so that it is less suitable for implantation. Common side effects of the Depo-Provera shot include changes to the menstrual cycle that can include abnormal, heavy or light bleeding. The Depo-Provera shot is not a good choice of birth control for anyone planning on getting pregnant within the next one to two years as it can take up to 18 months to become pregnant after using Depo-Provera.