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Different contraceptives work in different ways, they are used to prevent ovulation, they make sure that the ovum and the sperm do not meet, or even if they meet they aren't allowed to implant in the uterine cavity. Barrier methods include condoms; they do not allow the semen to deposit in the vagina, stopping it from going to the uterus and fallopian tube and lead to pregnancy. Then we also have the oral contraceptive pills that prevent ovulation, and the intra uterine devices prevent implantation where even after the fertilization of egg and sperm the fertilized zygote upon travelling to the uterine cavity is stopped by the device and is not allowed to implant. The contraceptives like DMPA that are injected prevent pregnancy by thickening the cervical mucus thickens and making the environment unfriendly to prevent implantation, some cases they may also prevent ovulation.