How to Rethink the Fourteen-Day Rule.

Recently, attention has been drawn to the basic principles governing the use of human embryos in research: specifically, the so-called fourteen-day rule. This rule stipulates that human embryos should not be allowed to grow in vitro past fourteen days of development. For years, the fourteen-day limit was largely theoretical, since culture techniques were not sufficient to maintain embryos up to this point. Yet in the past year, research has suggested that growing embryos beyond fourteen days might be feasible and scientifically valuable. At the same time, work with pluripotent stem cells, including human PSCs, has shown that under certain conditions, they can form structures that recapitulate developmental features of the postimplantation embryo. This raises the possibility that PSCs could generate embryo-like structures in vitro, even "synthetic embryos," that might provoke moral concern but would not fall under most current embryo research policies. In countries that permit embryo research, the fourteen-day rule has long been the linchpin of an effective policy compromise between what remain deeply divided moral positions on the human embryo's status. It has also, particularly in the United Kingdom, been influential in establishing a bioethics public-policy process. Any moves to change the rule must consider not just the implications for the use of embryos but also the potential impact of this model of bioethical governance of science.