u·ter·ine seg·ments(yū'tĕr-in seg'mĕnts)
1. Lower: the isthmus of the uterus, the lower extremity of which joins with the cervical canal and, during pregnancy, expands to become the lower part of the uterine cavity.
2. Upper: the main portion of the body of the gravid uterus, the contraction of which furnishes the chief expulsive force in labor.