To find out the length of the cervix, the difference between the results obtained using abdominal ultrasound and cervical ultrasound is usually between 2 and 5 millimeters. Abdominal ultrasound or cervical ultrasound is often used to examine a woman’s internal reproductive system. The length of the cervix using abdominal ultrasound is usually within a margin of error of 2 to 5 millimeters. The length of the cervix using pubic ultrasound is more accurate and can be considered error-free. In comparison, the length of the same woman’s cervix measured using abdominal ultrasound and cervical ultrasound may differ by 2 to 5 millimeters. Measurement of the length of the cervix is one of the common predictors of whether a woman is having a preterm labor, and in order to arrive at a more accurate value, the use of pubic ultrasound to measure the cervix is generally recommended in clinical practice.