Inability of the cervix to remain closed during pregnancy. The properly functioning cervix will be tightly closed until labor to keep the fetus safely inside the uterus. Some women have a cervix that does not stay closed and opens up in the second or early third trimester. This can result in miscarriage of the pregnancy or premature delivery of the baby. To avoid premature delivery the cervix may be stitched closed or the mother may be confined to bed. The reason for many of these cases is not known. However women who were exposed to diethylstilbestrol (DES) as a fetus or those who have had removal or cauterization of cervical tissue may be at increased risk for this.