Today's IUD's typically use a plastic T-shaped frame about 1¼" across and 1½" high with a small string on the base, and with copper or gold wrapped around the stem of this frame. The metal dissolves very slowly, and slow-release progestogen may be added as well.CDC These are spermicides with two functions - first to curtail the ability of sperm to swim in the cervical mucus thus blocking entry, or for sperm that do make it into the uterus, to terminate them before they can pass up a eustachian tube to the normal site of fertilization. Non-hormonal IUD's prevent implantation. Standard spermicide-hormonal IUD's prevent fertilization.
These IUDs put ions or hormone where they are functionally needed, not into systemic circulation, thus avoiding several problems caused by regular birth control pills. And in spite of the anomalous Dalkon Shield, IUD's have proved very safe for long-term use. They can also be used as "morning-after" treatment, and the non-hormonal form is compatible with breast feeding. The hormonal form of IUD is highly effective for up to seven years. Initial cost is high, but average cost is low.
Birth control pills contain estrogen and/or progestin, and their primary mode of action is to prevent egg formation and ovulation. They also alter cervical mucus making passage by sperm unlikely. So there is nothing to fertilize and usually nothing to do a fertilization. If a women is erratic about following the daily schedule of pills, though, she might ovulate and have a fertilization, in which case the birth control pills may block implantation of an embryo. The pills are far too weak to abort a fetus or cause separation of a placenta from the uterine lining. [The historic Catholic reason for not taking birth control pills is not avoidance of abortion as some claim.(see "Issues / Some Catholic History")]
A better reason for not taking birth control pills is their side effects and times of poor reliability. Perhaps as many as a third of women can not tolerate them, side effects sometimes including cysts on the ovaries or in the breasts and on rare occasions blood clots or even fluid over-pressure in the brain. The pills commonly cause loss of sexual interest after about five months of use. And doctors commonly forget to advise, even for single women, that birth control pills stop working during and following antibiotics. Protection does not resume until a wait time after the antibiotics that is as long as the duration of taking the antibiotics. Three weeks of antibiotics, for instance, means no protection for six weeks.
Natural birth control during lactation relies on similar hormones and has the same caveat. A women who provides all the nutrition for her child will not ovulate, not if she nurses every four hours during the day and every six hours at night. But as the child matures and she provides less, or perhaps even if she uses pumping instead of direct breast feeding, she will begin ovulating again. And she may have a fertilization she never notices, because with partial nursing, her partial supply of natural hormones may allow ovulation but prevent implantation.