(ACOG) adopted Boving's definition: "conception is the implantation of a fertilized ovum." The 1965 ACOG definition was imprecise because, by the time it implants, the embryo is called a blastocyst, so it was clarified in 1972 to "Conception is the implantation of the blastocyst." Some dictionaries continue to use the definition of conception as the formation of a viable zygote. Birth control – mechanism of action Birth control methods usually prevent fertilization. This cannot be seen as abortifacient because, by any of the above definitions, pregnancy has not started. However, some methods might have a secondary effect of preventing implantation, thus allowing the pre-embryo to die. Those who define pregnancy from fertilization subsequently may conclude that the agents should be considered abortifacients. Speculation about post-fertilization mechanisms is widespread, even appearing on patient information inserts for hormonal contraception, but there is no clinical support. One small study, using fourteen women, might be considered as providing evidence of such an effect for IUDs and a study of the combined oral contraceptive pill has been proposed. Possibly affected methods Hormonal contraception, including emergency contraception, are known to be effective at preventing ovulation. Some scientists believe hormonal methods may have a secondary effect of interfering with implantation of embryos. Intrauterine devices (IUDs) have been proven to have strong spermicidal and ovicidal effects; the current medical consensus is that this is the only way in which they work. Still, a few physicians have suggested they may have a secondary effect of interfering with the development of pre-implanted embryos; this secondary effect is considered more plausible when the IUD is used as emergency contraception. The lactational amenorrhea method works primarily by preventing ovulation, but is also known to cause luteal phase defect (LPD). LPD is believed to interfere with the implantation of embryos. Natural Family Planning (NFP) methods are intended to prevent fertilization through avoiding intercourse during fertile periods. Luc Bovens argues that, under an assumption that the age of gametes has an effect on embryo viability, errors in NFP method result in the occurrence of lower-viability embryos. This is intended to be an ethical thought experiment; Bovens states that his assumption "is not backed up by empirical evidence, but does have a certain plausibility." His argument is controversial. The age of gametes at the time of fertilization has been shown to have no effect on miscarriage rates in most cases, but is a significant risk