Since the first successful birth resulting from in vitro fertilization (IVF) in the late 1970s, human embryo research has been subject to limits of time and developmental benchmarks. National guidelines, laws, and international norms have prohibited scientists from culturing embryos for research beyond 14 consecutive days, or beyond the appearance of a structure called the primitive streak, which defines the beginning of the formation of the principal tissues of the body and the end of the period when an embryo can divide into identical twins (1). At the time this limit was put in place 40 years ago, there were no methods to culture embryos in a dish for anywhere close to 14 days.