Objectives To investigate the correlation between transperineal (TP) and transvaginal (TV) ultrasonography in the measurement of cervical length (CL) in the third trimester of pregnancy. Methods In this prospective study, CL measurements were conducted in women between 31 and 34 weeks of gestation by both the TP and TV approaches. A comparison of the measurements was made between the two techniques. The Pearson correlation coefficient, the paired samples t-test and Bland-Altman plot were used. Results In total, 240 women participated in the study. The CL was successfully measured transvaginally in all cases and transperineally in 229 (95.4%) of the cases. The mean TV CL was 32.8 ± 8.2 mm and the mean TP CL was 32.5 ± 8.1 mm. In the 229 cases with paired measurements, the Pearson correlation coefficient was 0.964. No significant differences in mean CL were identified between the two methods (t = 1.805; P = 0.072). In the Bland-Altman plot, the difference between the paired means was 0.26 mm and the 95% tolerance interval for any given paired observation (TV CL-TP CL) was -4.05 to 4.57 mm. Conclusion The findings of this study suggest that at 31-34 weeks of gestation the cervix can be visualized adequately by both the TV and TP sonography in about 95% of cases. The TP CL measurements demonstrate a close correlation with the TV measurements. TP ultrasound is a feasible alternative, especially in cases where the use of the vaginal transducer should be minimized or is not acceptable by women.