N. Tanmoun, K. Raungrongmorakot, P. Ruangvutilert, D. Boriboonhirunsarn, P. Tontisirin, and W. Butsansee, “Correlation of Cervical Length Obtained from Transperineal Ultrasonography, Transvaginal Ultrasonography and Digital Examination”, Thai J Obstet Gynaecol, vol. 15, no. 3, pp. 137–144, Apr. 2017.