External Link - Source: Science
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.