The embryo can be classified according to its age, its size or its morphologic characteristics. The correlation between these three criterias will allow identifying the embryonic Carnegie stages.
The sequence of appearance of the various structures in the development of the embryo remains always the same. Nevertheless embryos of the same age can be in various conditions of development. It is thus practical to divide the development into stages that take inner and outer morphologic signs into consideration. In this way precise statements can be made and various embryos can be compared with each other in a meaningful way.
This separation into stages was originally developed by Streeter (1942) who termed the various organizational stages "horizons". Later this scheme was completed by O'Rahilly and Müller (1987) who spoke more simply of embryonic stages or Carnegie stages. These stages permit the embryologist to make a more precise classification than just using size would allow (nevertheless, one can say that the relation between the stages and size remain relatively constant). Moreover, with the stage classification more precise statements can be made than is possible only using the age of the embryo. Chronological statements are really less accurate. Among other things, this is because the date of ovulation is often calculated based on the date of the last menstruation. Deriving the moment of ovulation from the menstruation date, though, is imprecise (+/- 2 days).
Taking into consideration various external and internal landmarks of embryonic development, it was decided to divide the 8 embryonic weeks (56 days) into 23 Carnegie stages.
The fetal period that begins after the 8th week is characterized by the growth and maturation of the organs. The inner and outer morphologic alterations are less noticeable. For this reason one no longer divides the fetal period into Carnegie stages.