Why dose an incompetent cervix give way from 12 to 14 weeks on?
In the previous section, I mentioned that the cervix is made up of only about 10 percent muscle; the rest of it is a fibrous material that can stretch and soften in pregnancy. The cervix opens at ovulation allowing the entrance of sperm, but apart from that time it is normally closed.
During pregnancy, the cervix softens slightly, but otherwise it remains tight and is filled with a very thick or viscid mucus plug that stays in place until very late in pregnancy. At that point the cervix starts to shorten, or efface, because it is beginning to open so the baby’s head can go through it. The cervix must remain tightly closed to protect the fetus and uterine contents from the introduction of infection from the outside or the vagina.
The part of the uterus above the cervix is made up mainly of strong muscle. During early pregnancy, the uterus increases in size due to pregnancy hormones, which cause muscle fibers in the body of the uterus to increase in number and to lengthen. Then, from the middle of the second trimester, about the 12th to 14 week on, the uterus also gets bigger because the growing fetus and amniotic sac now push up and cause its expansion. This is what normally happens before delivery. The uterus gets large, and at around 40 weeks, the strong muscle helps expel the baby through the cervix opening.
However, if the cervix is weak, or incompetent, or if the uterus is misshapen, this normal sequence of events does not occur. When there is an abnormality, the muscle in the uterus is often replaced by fibrous tissue that cannot expand. As the baby grows and pushes up on the top of the uterus (fundus), the uterus refuses to give because of the fibrous tissue, and the pregnancy begins to act like a metal rod—pushing down on the cervix, which will start dilating. This will precipitate a rupture of the membranes, and a rapid miscarriage follows.
Symptoms are very few. But, if you do notice that your uterus is contracting and becoming very hard at this time, if there is a very heavy mucus discharge or any vaginal bleeding, you should report to your doctor. Often no pain is experienced until the miscarriage is already well advanced. However, pressure on the cervix may produce a pain in the vagina that has been described as “like a knife pushing upward from the vagina into the pelvis.” Severe backache may also occur. Any such symptoms around this time of pregnancy should be immediately reported.
However, if a diagnosis has been made and a stitch can be put in at the correct time, the success rate for producing a healthy term baby is very good, at least 80 to 90percent. New studies have shown that the earlier in pregnancy the stitch is put in, and the higher upon the cervix your doctor can place it, the more effective is its hold. You and your doctor will be doing your best to stimulate nature. The cervix needs to be closed and tight to maintain a pregnancy.
Sometimes an incompetent cervix, especially one that happened beyond about 20 weeks, may be associated with uterine contractions. It is not known whether this is true premature labor, or whether the weak cervix is causing the contractions. Nevertheless, at this late stage in addition to the stitch, your doctor may put you on the drug ritodrine to prevent further contractions. The drug is safe to take in pregnancy;  its only side effect on you is a rapid heart beat.