From the Research
During labor progression, the upper uterine segment thickens while the lower uterine segment thins, reflecting the coordinated process of effective contractions, as evidenced by a study published in the American Journal of Obstetrics and Gynecology in 2003 1. This change in thickness is a result of the muscle fibers in the upper segment shortening and retracting with each contraction, generating the force needed to push the fetus downward.
- The upper segment progressively thickens from about 5-7mm to 10-12mm as labor advances.
- The lower segment gradually thins from about 7-9mm to 2-3mm and stretches to accommodate the descending fetus, forming the birth canal. This inverse relationship between segments creates a physiological gradient that facilitates fetal descent.
- Ultrasound assessment of these thickness changes can help clinicians evaluate labor progress, with a thickening upper segment and thinning lower segment indicating effective contractions and normal labor progression.
- Abnormal patterns, such as minimal changes in thickness between segments, may suggest dysfunctional labor requiring intervention, as supported by a study published in the same journal in 2016 2. The most recent and highest quality study, published in 2016, found that the inclusion of lower uterine segment thickness measurement in the decision of the route of delivery allows a low risk of uterine rupture during trial of labor after cesarean delivery 2.
- This study recruited 1856 women and found that the rate of trial of labor was 9%, 42%, and 61% in the high, intermediate, and low risk categories, respectively.
- There were no symptomatic uterine ruptures among the 984 trials of labor, which is a rate lower than the expected 0.8%. Therefore, monitoring the changes in upper and lower uterine segments' wall thickness is crucial in reflecting labor progression and identifying potential complications, and should be prioritized in clinical practice, as supported by the highest quality evidence available 2.