Gramellini, D., et al. “Cervical Lenght As a Predictor of Preterm Delivery: Gestational Agerelated Percentiles Vs Fixed Cut-Offs”. Acta Biomedica Atenei Parmensis, vol. 78, no. 3, Dec. 2007, pp. 220-4, https://mattioli1885journals.com/index.php/actabiomedica/article/view/1906.