Even when the precise date of ovulation or fertilization is known, individual embryos develop at different rates and so it is not possible to make an exact correlation between age or length and stage of development. To reflect this variability, developmental stages have been defined according to morphological criteria.
The embryonic period has been subdivided into 23 Carnegie stages. Each stage has a specific duration according to the occurrence of morphological features. It is possible to spot these features due to recent advances in ultrasound technology. We follow the time table of O'Rahilly and Müller, 2001, Human Embryology and Teratology, Wiley-Liss 2001, third edition.
Until now, it was not possible to describe development during the fetal period in terms of stages; criteria such as age, weight and body length have been used. However, weight and body length vary considerably between individual fetuses, and there are various methods for measuring the body length.
The following table of embyonic stages is based on O'Rahilly and Müller 2001 (see above).