Roughly 50 years ago, scientists pursued a line of inquiry about the creation and development of human embryos in the lab. As of now, the extent of this research is limited to the first 14 days following fertilization — 14 days after one cell from mom joins one cell from dad to create a cell whose genetic composition has never before been seen in human history. 

This 14-day limit originates from a 1979 report produced by the Ethics Advisory Board of the U.S. Department of Health, Education, and Welfare. The report states that “it is acceptable from an ethical standpoint to undertake research involving human in vitro fertilization and embryo transfer provided that … no embryos will be sustained in vitro beyond the stage normally associated with the completion of implantation (14 days after fertilization).”

Since the report was published, researchers who studied these two-week-old human embryos discovered that 14 days is also about the time when a structure called the primitive streak forms inside the embryo. This structure delineates the central axis of the body, separating right from left. To quote the authors of a

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