As work in real and model embryos movesforward, scientists are keen to know how similar the two really are. Finding out how models differ in their molecular details, and how their cells behave, is the main reason researchers wish to push beyond 14 days in real embryos. “We can learn a lot from a model,” says Jesse Veenvliet, a developmental biologist at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany. “But it’s important to know where it goes wrong.”

Researchers are now permitted to grow human embryos in the lab for longer than 14 days. Here’s what they could learn.