Objective: To evaluate the accuracy of cervical effacement reported as a percentage by digital cervical exams using cervical length determined by transvaginal ultrasonography as a standard. Methods: Records of pregnant women who had a digital cervical exam and subsequent transvaginal ultrasound scan for cervical length between January 2005 and December 2008 were reviewed. Digital cervical exams were performed by different examiners. Transvaginal ultrasound was performed by one examiner who did not perform any of the digital exams. Cervical effacements were recorded as a percentage and cervical lengths were measured in centimeters. Results: A total of 173 women met the study criteria. Average cervical length for 0% effacement was 3.3 ± 1.1 cm (0.8 - 5.0 cm); 20% effacement, 1.6 ± 1.0 cm (0.9 - 3.0 cm); 25% effacement, 2.2 ± 0.2 cm (2.0 - 2.3 cm); 30% effacement, 2.6 ± 0.4 cm (2.1 - 3.0 cm); 40% effacement, 3.0 ± 0.4 cm (2.6 - 3.4 cm); 50% effacement, 2.4 ± 1.1 cm (0.6 - 4.6 cm); 60% effacement, 2.3 ± 1.4 cm (0.7 - 4.3 cm); 70% effacement, 2.2 ± 0.8 cm (1.1 - 3.3 cm); 75% effacement, 1.7 ± 1.4 cm (0.7 - 2.7 cm); 80% effacement, 2.0 ± 0.9 cm (0.6 - 4.4 cm); 90% effacement, 0.7 ± 0.4 cm (0.4 - 0.9 cm); 100% effacement, 1.2 ± 1.5 cm (0.3 - 3.0 cm). The coefficient of variation ranges from 10% - 124%. Conclusion: The traditional method of reporting cervical effacement as a percentage is unacceptably inaccurate compared to the actual cervical length determined by vaginal probe ultrasound.