Objective: To evaluate the cervical length by means of transvaginal ultrasonography in a population of healthy pregnant women, developing a normality curve in the period between the 20th and 34th gestational weeks. Materials and methods: Cross sectional study involving 145 healthy singleton pregnancies with healthy fetuses between 20 and 34 weeks of gestation, evaluated at the units of ultrasonography of Hospital Universitário Júlio Müller and of a private center. Each pregnant woman underwent a single transvaginal ultrasonography study, with measurement and recording of the cervical length. Exclusion criteria were: preterm labor, preterm premature rupture of membranes, placenta previa, use of tocolytics and/or progesterone, cerclage or any surgical intervention prior to pregnancy. The association between cervical length and gestational age was evaluated by linear regression analysis. Results: The cervical length presented a progressive decrease (0.8 mm/week) as the gestational age progressed (r = –0.351; p < 0.001). The women with cervical funneling presented shorter cervix (p = 0.001). Interpolation of the 5th, 50th and 95th percentiles provides a graph to be used as a reference. Conclusion: The mean cervical length decreases about 0.8 mm per week in the period between the 20th and 34th gestational weeks.

Keywords: Cervical length measurement; Ultrasonography; Cervix uteri; Preterm labor.