First-Trimester Embryology: An Overview

Normal human development is a continuum. However, unfortunate interruptions in this ongoing process can result in abnormal development and subsequent congenital anomalies. These anomalies can be the result of many etiologic factors. Approximately 3 % or greater of pregnancies are complicated by congenital anomalies. The purpose of this chapter is to provide the clinician or sonographer with the essential basics of embryological and fetal development, to better understand mechanisms of normal and abnormal first-trimester human development. Understanding of these mechanisms is imperative for adequate evaluation of the first-trimester fetus in the ultrasound laboratory.