Researchers at the University of Cambridge, UK have succeeded in reconstructing mouse embryos by combining pluripotent embryonic and multipotent trophoblast stem cells in a 3D scaffold; the study from the laboratory of Professor Zernicka-Goetz, recently published in Science, provides a break-through tool to probe early mammalian development outside the uterus. Achieving a similar feat with human cells might necessitate reconsideration of the 14-day rule as a limitation of such research.