Ultrasound has the potential for diagnosing fetal abnormalities in utero with a high degree of accuracy. During the first trimester, peculiarities of fetal development may suggest to the novice sonographer an abnormality where one does not exist. Similarly, particular anatomic structures may appear normal in the first trimester, yet evolve into an obvious abnormality later in gestation. For these reasons, the ultrasonic diagnosis or exclusion of a congenital anomaly in the first trimester should be made only with caution.