Two groups have reported that they have sustained human embryos in vitro for 12–13 days. Embryos normally implant in the wall of the uterus at around day seven. Until now, no one had reported culturing human embryos in vitro beyond nine days, and rarely have they been sustained for more than seven.
In principle, these two lines of research could lead to scientists being able to study all aspects of early human development with unprecedented precision. Yet these advances also put human developmental biology on a collision course with the ’14-day rule’ — a legal and regulatory line in the sand that has for decades limited in vitro human-embryo research to the period before the ‘primitive streak’ appears. This is a faint band of cells marking the beginning of an embryo’s head-to-tail axis.
The 14-day rule has been effective for permitting embryo research within strict constraints — partly because it has been technologically challenging for scientists to break it. Now that the culturing of human embryos beyond 14 days seems feasible, more clarity as to how the rule applies to different types of embryo research in different jurisdictions is crucial.
The 14-day limit was first proposed in 1979 by the Ethics Advisory Board of the US Department of Health, Education, and Welfare. It was endorsed in 1984 by the Warnock committee in the United Kingdom, and in 1994 by the US National Institutes of Health’s Human Embryo Research Panel. In at least 12 countries, this limit is encoded in laws governing assisted reproduction and embryo research. The rule is also embodied in numerous reports commissioned by governments, and in scientific guidelines for embryo and assisted-reproduction research. These include China’s 2003 Ethical Guiding Principles on Human Embryonic Stem Cell Research and India’s 2007 Guidelines for Stem Cell Research and Therapy.