Setting the development of embryos on halt until the time is right to give birth sounds like something out of a sci-fi novel, but for many mammals what’s called “embryonic diapause” is a vital element of developing their newborns. Even though experts have recognized since the 1850s that some animals have this ability, it is only now becoming clear how it could teach us valuable lessons about stem cells, cancer, and human pregnancy. The main benefit of pausing pregnancy is that it separates mating and birth. There are two main ways in which animals do this. Scientists now believe this could lead to new treatments to prevent cell division or to identify markers for tumor stem cells.