Verrotti, C., E. Caforio, and D. Gramellini, et al. 2007. “Ultrasound Screening in Second and Third Trimester of Pregnancy: An Update”. Acta Biomedica Atenei Parmensis 78 (3): 229-32. https://mattioli1885journals.com/index.php/actabiomedica/article/view/1908.