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incompetent cervix a uterine cervix abnormally prone to dilate in the second trimester of pregnancy, resulting in premature expulsion of the fetus. cervix uÂ´teri , uterine cervix the narrow lower end of the uterus, between the isthmus and the opening of the uterus into the vagina.
In the cervix is a tiny opening, about the size of a pinhead, called the external os. The external os is filled with a thick plug of protein (mucus) that serves as a barrier to the entrance of the uterus. The cervix leads into a smooth-muscle-walled tube called the vagina, or birth canal.