Introduction

In 2016, two independent teams, one in the United States and one in the United Kingdom, reported culturing human embryos in vitro up to fourteen days after fertilization. The experiments were terminated by day fourteen or at the formation of the primitive streak to respect an international norm and, in the case of the U.K. team, to be in compliance with the law. This norm is known as the fourteen-day rule or fourteen-day limit.

The fourteen-day limit for embryo research was proposed first in a 1979 U.S. Department of Health Education and Welfare (DHEW) report and later in the 1984 U.K. Report of the Committee of Inquiry into Human Fertilisation and Embryology, commonly known as the Warnock Report. The preparation of both documents involved extensive public and stakeholder engagement, including in-person meetings and the review of hundreds of letters from the public. The reports focused on the clinical practice of in vitro fertilization (IVF) and its related research. 

Fourteen days was not chosen arbitrarily. Rather, specific reasons were offered for adopting it within these reports, and additional justifications were articulated in the three decades following their release. First, the primitive streak appears around fourteen days, marking the first visible sign of significant organization of the embryo just prior to neural tube formation, and it is an event that can be easily identified in culture. Furthermore, twinning does not seem to occur after fourteen days, suggesting that this is the point of individuation, leading some to claim that destruction of embryos prior to this point does not destroy individuals.

The diverse international regulations on human embryo research adopted since the limit was proposed reflect different normative views. Some countries (including Austria, Germany, Italy, Russia, and Turkey) ban human embryo research. The United Kingdom includes the fourteen-day limit in the Human Fertilisation and Embryology Act of 1990, which permits human embryo research after approval by a regulatory authority. Other countries (including Canada, South Korea, and Sweden) allow certain human embryo research but have enshrined the fourteen-day limit into law. Still other countries (Israel among them) regulate human embryo research but do not specify a particular limit. In the United States, researchers cannot use federal funds for human embryo research, but privately funded research beyond fourteen days is possible. Nevertheless, to date, scientists appear to respect the fourteen-day limit, regardless of its legal status in their country.

The fourteen-day limit is often framed as an ethical compromise to permit human embryo research despite the moral objections of some. However, when the limit was established, it was not technically possible to culture human embryos beyond five or six days. Thus, the fourteen-day limit imposed no tangible restriction to human embryo research, and proponents of this research lost little in agreeing to it. A policy that prohibits what is not scientifically feasible does not impinge on practice. In contrast, those who opposed all human embryo research relinquished their preference for a complete prohibition. Only now that technology has advanced does the fourteen-day limit appear to reflect a compromise between some human embryo research proponents and opponents.

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