Quality Requirements for Ultrasound Examination in Early Pregnancy (DEGUM Level I) between 4 + 0 and 13 + 6 Weeks of Gestation. DEGUM Level III Group Recommendations of the Section Obstetrics and Gynecology, Deutsche Gesellschaft für Ultraschall in der Medizin (DEGUM) - Rempen, A; Chaoui, R; Häusler, M (Ultraschall Med, 2016)