physiological retraction ring

physiological retraction ring

a ridge around the inside of the uterus that forms during the second stage of normal labor at the junction of the thinned lower uterine segment and thickened upper segment. It forms as a result of progressive lengthening of the muscle fibers of the lower segment and concomitant shortening of the muscle fibers of the upper segment. Compare constriction ring, pathological retraction ring.
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