The following comes from a May 4 Science Magazine article by Patrick Monahan:

It’s easy to obey a rule when you don’t have the means to break it. For decades, many countries have permitted human embryos to be studied in the laboratory only up to 14 days after their creation by in vitro fertilization. But—as far as anyone knows—no researcher has ever come close to the limit. The point of implantation, when the embryo attaches to the uterus about 7 days after fertilization, has been an almost insurmountable barrier for researchers culturing human embryos.

Now, two teams report growing human embryos about a week past that point. Beyond opening a new window on human biology, such work could help explain early miscarriages caused by implantation gone awry. As a result, some scientists and bioethicists contend that it’s time to revisit the so-called 14-day rule.

“We’re here sooner than we thought,” says Insoo Hyun, a bioethicist at the School of Medicine at Case Western Reserve University in Cleveland, Ohio, and co-author of a commentary in Nature this week calling for a reassessment of the 14-day rule. About 4 years ago, a group led by Magdalena Zernicka-Goetz, a developmental biologist at the University of Cambridge in the United Kingdom, first reported culturing mouse embryos past the implantation stage, and it has been improving its methods ever since.

The latest tricks also work with human embryos, Zernicka-Goetz’s team and a collaborating group led by Ali Brivanlou, a stem cell biologist at The Rockefeller University in New York City, report this week in Nature and in Nature Cell Biology.

The teams halted their studies when the embryos were 14 or 13 days past fertilization, as U.K. laws require and several U.S. guidelines recommend. But that was enough time for them to see that mouse embryos are imperfect models for human ones. For example, the cells that develop into the fetus and the yolk sac diverge later in human embryos. “You have to study the human embryo to understand the human embryo,” Zernicka-Goetz says.

The 14-day rule is pegged to the time at which human embryos develop the “primitive streak”—an easily identifiable group of cells that appears when embryos can no longer fuse or split. That, in the eyes of some religious bioethicists, marks the threshold at which an embryo is a distinct human.

Now that culture methods have finally caught up to the ethics of the 14-day rule, Hyun and his commentary co-authors say it’s time to start a new conversation about whether there is scientific need, as well as a broader consensus, for lengthening the time human embryos can be grown in the lab. The 14-day rule was conceived as a policy tool to enable research, he says. “It shouldn’t be thought of as a hard and fast moral pronouncement.”