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a pathologic retraction ring at the juncture of the upper and lower segments of the uterus occurring at any stage of dysfunctional or obstructed labor. Normally, as contractions progress during labor the upper segment of the uterus becomes thicker and more active while the lower segment becomes thin-walled, supple, and passive. The boundary between the two points is marked by a normal physiologic retraction ring. In difficult labor the ring can become hardened and more prominent. The pathologic ring is observed as an indentation in the abdomen as the ring rises against the abdominal wall. The uterine wall below the pathologic ring becomes thin and distended as labor continues. If obstruction to the passage of the fetus and placenta is not relieved, the lower segment of the uterus may rupture.