Why this lab-grown human embryo has reignited an old ethical debate

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. But that won’t be welcomed by those who consider the rule to have a firm moral grounding—or by those who oppose any research on human embryos.

“We’re here sooner than we thought,” says Insoo Hyun, a bioethicist at the School of Medicine at Case Western Reserve University...