UDs come in a number of forms. They can be inert, or have copper or hormones embedded within them. Most scientists believe that they interfere with the ability of sperm to get to an egg in time to fertilize it before they die.
Research does not support the idea that they prevent fertilized eggs to implant. The journal Fertility and Sterility published a study in 1985 that followed three groups of women for 15 months. One group had an IUD, one group had their tubes tied, and one group was trying to get pregnant. They then measured hormone levels to see if fertilization occurred. It did so only in the group trying to get pregnant.
Another study found that a telltale sign of fertilization ? a surge of the hormone human chorionic gonadotropin ? occurred in only 1 percent of 100 cycles in women using IUDs. This would be consistent with the failure rate of IUDs in general. In other words, IUDs do not appear to work by aborting a fertilized egg.
Emergency contraception, which is really just a large dose of the hormones in a birth control pill, works in a similar manner. The pills can thicken the mucus in the cervix to make it difficult for sperm to reach the egg, and they prevent ovulation from occurring in the first place. Because the doses of medication are very short-term, they probably cannot affect the uterine lining in such a way as to affect implantation.
Moreover, the fact that both of these forms of contraception can fail, and allow pregnancies to occur, provides evidence that if a fertilization occurs, it can move on to implant and grow
http://www.nytimes.com/2012/06/06/h...antation-science-suggests.html?pagewanted=all