Scientific panel loosens ’14-day rule’ limiting how long human embryos can be grown in the lab

An influential scientific panel cracked open the door on Wednesday to growing human embryos in the lab for longer periods of time than currently allowed, a step that could enable the plumbing of developmental mysteries but that also raises thorny questions about whether research that can be pursued should be.

For decades, scientists around the world have followed the “14-day rule,” which stipulates that they should let human embryos develop in the lab for only up to two weeks after fertilization. The rule — which some countries (though not the United States) have codified into law — was meant to allow researchers to conduct inquiries into the early days of embryonic development, but not without limits. And for years, researchers didn’t push that boundary, not just for legal and ethical reasons, but for technical ones as well: They couldn’t keep the embryos growing in lab dishes that long.

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