Carnegie Stages

Historically the exact age of human embryo specimens has long perplexed embryologists. With the menstrual history of the mother often unknown or not exact, and the premenstrual and postmenstrual phases varying considerably among women, age sometimes came down to a best guess based on the weight and size of the embryo. Wilhelm His was one of the first to write comparative descriptions of human embryos in the late 1800s. Soon afterward, Franklin P. Mall, the first director of the Carnegie Institution of Washington's (CIW) Department of Embryology, expanded upon His' work. Mall's first efforts were to place embryos into stages based on menstrual ages and body length. This method ran into problems however when it became apparent that obtaining menstrual ages was often impossible or simply too inaccurate even if the information could be obtained from the women who carried the embryos. Mall decided instead to look for patterns among embryos to come up with some type of staging system whereby embryo age could be more accurately determined.