 The Carnegie Staging System, CSS, is widely accepted as a gold standard for embryological development. However, there have been inconsistencies in the data collected from different sources, leading to large variations in the reported ages and lengths of embryos. The most reliable source of CSS data is the work of Professor Hill, who developed a more accurate method of measuring embryonic ages and lengths. This method is now considered the gold standard for CSS data. This article was authored by Sander Fleierman, Melanie Tidsterman, Melek Rauschen, and others.