Millennials are increasingly making time in their busy schedules to put their eggs on ice. More effective flash-freezing technologies, micro-optimized ad targeting, and a growing willingness among companies to follow Silicon Valley's lead of including fertility treatments in benefit packages have made the practice more attractive to would-be parents. At last count, more than 65,000 so-called social egg freezers in the US are doing so each year, despite the costs, physical toll, and the uncertain odds. Egg freezing doesn't guarantee a child-bearing future; recent data from one of Europe's largest IVF clinics indicates that while most eggs survive the thawing process, only about a third of them result in successful pregnancies. To give patients a sense of what their chances are, fertility doctors use a system taken from the annals of 19th-century actuarial science.