Fetal and perinatal medicine is a rapidly expanding field, and noninvasive imaging by means of ultrasonography and MRI is playing a major role in refining diagnosis and therapy. Recent technological advances in these imaging modalities now allow unprecedented morphological depiction of the fetus and excellent insight into complex pathologic conditions, as well as yielding superior guidance for therapeutic fetal inter ventions. I am very pleased that Professor F. Avni , a leading international pediatric radiologist, was prepared to take on the challenging task of preparing and editing this comprehen sive and up-to-date overview of our knowledge in the area of fetal and perinatal imaging. He has been successful in engaging well-known experts with outstanding qualifications in fetal imaging to join him in this venture. I would like to congratulate Professor Avni and all contributing authors most sincerely for their excellent work. I am confident that this outstanding volume will meet with great interest not only from general as well as specialized pediatric radiologists but also from neonatologists and pediatricians. I trust it will enjoy the same success as many previous volumes in this series. ALBERT L. BAERT Leuven Preface Fetal and perinatal medicine would not have developed without the extensive use of obstetric ultrasound (US). In order to be efficient, the examination has to be performed very carefully and by sonologists fully conversant with the normal and abnormal development of the fetus.