Objective: The remodeling of uterine connective tissue during labor can lead to the reorganization of the extracellular matrix that, in turn, may influence the biomechanical properties of the myometrial wall. We hypothesized that the stretching of the lower uterine segment in laboring women with dystocia changes the viscoelastic properties of the uterine wall. Study design: We tested the tensile strength of lower uterine segment myometrium in 68 pregnant women at term. The biomechanical, structural, and biochemical properties were compared among 3 groups: (1) 39 laboring women who underwent primary low- transverse cesarean delivery for labor dystocia, (2) 12 nonlaboring women who underwent primary elective low- transverse cesarean delivery and (3) 17 women who underwent an elective repeat low- transverse cesarean delivery at term. The tensile properties were quantitated with a stretching regimen that was designed to mimic the conditions of labor. Parameters such as slope, yield point, and break point were recorded, analyzed, and interpreted. Biochemical properties were determined by the measurement of the sulfated glycosaminoglycans, hydroxyproline, and pyridinoline- deoxypyridinoline. Histologic properties of the connective tissue were assessed by collagen birefringence. Lastly, the association between these properties and biomechanical responses were compared among groups. Results: Lower uterine segment myometrium specimens obtained from laboring women were stiffer compared with specimens from women who were not in labor (P = .013) or had scarred myometrium (P < .001). The force that was required to reach the yield point was similar between labor and nonlabor groups (P = .216). Likewise, a previous lower uterine segment scar did not alter the yield point. The break point was similar among all groups (P = .317). Sulfated glycosaminoglycan levels were unaffected by labor or scarring (P = .354). Scarred lower uterine segment myometrium had a higher collagen content compared with unscarred myometrium specimens that were obtained during labor (P = .025). Although there were similar degrees of collagen cross- linking among groups (P = .212), there was lower collagen birefringence in myometrium from laboring women compared with nonlaboring women (P < .001). Conclusion: Labor alters the viscoelastic properties of myometrium. Lower uterine segment myometriumis stiffest in women with dysfunctional labor compared with nonlabor control subjects. Labor and scarring also alter the pattern of collagen birefringence. Similar collagen cross- linking among the study groups may explain the reason that the breaking strength of the tissue is not altered by the state of labor and the reason that the rupture of the uterine scar is a rare event. Objective: The remodeling of uterine connective tissue during labor can lead to the reorganization of the extracellular matrix that, in turn, may influence the biomechanical properties of the mytrial wall. We hypothesized that the stretching of the lower uterine segment in laboring women with dystocia changes The biomechanical, structural, and biochemical properties were among among groups: (1) 39 laboring women who underwent primary low-transverse cesarean delivery for labor dystocia, (2) 12 nonlaboring women who underwent primary elective low-transverse cesarean delivery and (3) 17 women who underwent an elective repeat low- transverse cesarean delivery at term. The tensile properties were quantitated with a stretching regimen that was designed to mimic the conditions of labor. Parameters such as slope, yield point, Biochemical properties were determined by the measurement of the sulfated glycosaminoglycans, hydroxyproline, and pyridinoline-deoxypyridinoline. Histologic properties of the connective tissue were assessed by collagen birefringence. Lastly, the association between these properties and Results: Lower uterine segment myometrium specimens obtained from laboring women were stiffer compared with specimens from women who were not in labor (P = .013) or had scarred myometrium (P <.001). The force that was required to reach the yield point was similar between labor and nonlabor groups (P = .216). Also, the previous point uterine segment scar did not alter the yield point. . Sulfated glycosaminoglycan levels were unaffected by labor or scarring (P = .354). Scarred lower uterine segment myometrium had a higher collagen content compared with unscarred myometrium specimens that were obtained during labor (P = .025). Although there were similar degrees of collagen cross linking among groups (P = .212), there was lower collagen birefringence in myometrium from laboring women compared with nonlaboring women (P <.001). Conclusion: Labor alters the viscoelastic properties of myometrium. Lower uterine segment myometriumis stiffest in women with dysfunctional labor compared with nonlabor control subjects. Labor and scarring also alter the pattern of collagen birefringence. the study groups may explain the reason that the breaking strength of the tissue is not altered by the state of labor and the reason that the rupture of the uterine scar is a rare event.