ABSTRACT
This chapter describes ultrasound findings in early pregnancy, respectively, normal development of human embryo and fetus from the third until the tenth week after conception. The most widely used method for the classification of embryos in embryology is the Carnegie staging system. This system was developed at the Carnegie Institute, which began collecting and classifying embryos in the early 1900s. An embryo is assigned a Carnegie stage (numbered from 1 to 23) based on its external and internal morphological features. Age and size of embryo have been demonstrated to be of limited value in the classification of embryos as they quite frequently overlap in several stages. The staging and description of age in traditional embryology are different from that used in obstetrics. Gestational age in obstetrics and fetal medicine is based on menstrual age rather than conceptual age. Menstrual age was chosen for this chapter, because this is the parameter widely used in clinical medicine. According to the menstrual age, the embryonic period includes the first 10 weeks of pregnancy; however, due to its clinical relevance the 11th and 12th weeks of fetal development were included in this chapter.
