There are several factors which can affect contraceptive effectiveness in preventing pregnancy. Different types of contraceptives have different basic rates of success when used correctly. The effectiveness of each type depends on people using them as they were intended; human error plays a large role in contraceptive effectiveness and is often the reason for failure. Some types of contraceptives, such as condoms or diaphragms, may simply malfunction, while others can be adversely impacted by other behaviors that the users engage in.
Even when they are all used as intended, overall contraceptive effectiveness can vary greatly among the different methods. The only method that can completely prevent pregnancy is abstinence. IUDs, Depo Provera shots, and tubal ligation or vasectomy are typically the next most effective, with birth control patches, rings, and implants offering only slightly lower rates of success. Some methods that tend to fail more frequently include natural family planning and barrier methods like condoms, diaphragms, and sponges.
Rates of success are based on perfect use; contraceptive effectiveness is often strongly influenced by how well the person using it follows the instructions. Women taking birth control pills need to take them every day at the same time for them to work correctly; those who take them at different times of day or miss doses are more apt to get pregnant. Natural family planning methods may fail if a woman does not estimate her fertile days accurately or a man fails to withdraw prior to ejaculation. Some people may just fail to use their chosen method consistently.
Although a contraceptive may typically be very effective and the person using it may do everything correctly, there is always the possibility that it may just malfunction. Condoms may break during intercourse, allowing semen to enter the vagina. Barriers like diaphragms and cervical caps may be moved during sex; IUDs can become dislodged and be ejected from the uterus.
Certain drugs can interfere with birth control pills, making them less effective, and women who take them at the same time increase their risk of getting pregnant. It is possible to mitigate these problems, particularly when the risk is known, as with taking medications that affect birth control pills. Contraceptive effectiveness can easily be improved by using a secondary method, such as using condoms with birth control pills or a spermicide with a diaphragm.