The overlooked impact of lower uterine segment contractions on second trimester cervical assessment

Introduction: Asymptomatic contractions in the lower uterine segment (LUS) may affect uterocervical angle and cervical length; however, this has never been investigated. Therefore, the aim of this study was to evaluate the effect of LUS contractions on uterocervical angle, cervical length and LUS thickness.

Materials and methods: This was a prospective, observational, single-cohort study conducted on 102 asymptomatic singleton pregnancies between 19.0 and 22.6 weeks of gestation. Uterocervical angle and cervical length were measured by transvaginal ultrasound at two different time points with an interval of at least 20 minutes. LUS thickness was also measured as an indirect marker of myometrial contraction. A linear and curvilinear regression were performed to explore a potential association between LUS contractions, measured as increments in LUS thickness, and changes in cervical length and uterocervical angle.

Results: The linear regression between changes in LUS thickness and changes in cervical length showed that LUS contractions impact CL; for every one-millimeter increase in LUS thickness, cervical length increased by 0.909 mm, when the isthmus was included in the cervical length measurement (R2=0.358; β=0.909; P<0.001). By contrast, when the isthmus was not included in the measurement, a curvilinear relation between changes in LUS thickness and changes in cervical length was found (R2=0.077; β1=0.575, β2=0.038; P=0.018). The relation between changes in uterocervical angle and changes in LUS thickness was not significant.

Discussion/conclusion: LUS contractions can be observed in many asymptomatic women during the second trimester of pregnancy. LUS contractions lead to an increase in cervical length and LUS thickness, thus impacting ultrasound cervical assessments. These contractions do not affect the uterocervical angle.