Objective: To determine whether the transvaginal ultrasonographic measurement of the cervical volume at 19–22 weeks could predict a post-term pregnancy. Materials and methods: This work involves a retrospective case–control study comprising 44 women who delivered beyond 41 weeks and 87 women who delivered at term (37–40 + 6 weeks), matched by age and parity. All of them had undergone cervical length measurement and cervical volume estimation at 19–22 weeks. Results: Patients’ median of age was 35 years in term gestations and 34.5 years in prolonged pregnancies (p = 0.313). The mean of gestational age during delivery in the term gestation group was 275.41 days vs 289.34 days on prolonged gestations (p < 0.001). We did not observe differences in the mean cervical volume between term delivery (37.37 cm³, 95% CI: 34.59–40.14) and those who had post-term delivery (38.06 cm³, 95% CI: 33.34–42.77) (p = 0.788). In addition, we did not find differences in the median cervical length (39.0 mm vs 37.0 mm) (p = 0.610). Conclusion: It seems that there is no relationship between the cervical volume measured in the ultrasound of 20-week gestation and the prolongation of pregnancy beyond week 41.