An incompetent cervix will cause miscarriage at about the 20th week of pregnancy.
The canal of the cervix is about 2-5 cm. in length narrowing at its lower end where it joins the vagina (the external cervical os) and at its upper end where it joins the body of the uterus (the internal cervical os). During pregnancy, while the uterus is enlarging the cervix must remain closed otherwise the foetus would fall through into the vagina. There are many circular muscle fibres surrounding the cervix, especially the internal cervical os, which normally keep it tightly closed throughout pregnancy despite the continuous recurrence of Braxton Hicks’ contractions.
The muscle surrounding the internal. os may have been damaged so that it is incapable of keeping the cervix closed during pregnancy. The internal cervical os will then start to open soon after the 14th week, and by about the 20th week of pregnancy the cervix will be about 1 in. dilated. At this stage the membranes, or bag of waters, will bulge through the cervix into the vagina and will eventually break. There will be a sudden loss of a large quantity of water from the uterus and the vagina followed by miscarriage which usually occurs fairly quickly with comparatively little discomfort.
The main causes of incompetent cervix are:
Injury. During a particularly difficult or rapid labour, or where a baby is unduly large, the muscle fibres of the internal cervical os may occasionally be damaged and the next pregnancy miscarries at about the 20th week.
Operative. When the cervix is injured at an operation for the treatment of painful periods, or to perform an abortion, the circular musle fibres around the internal cervical os may be damaged sufficiently to render the cervix incompetent in the next pregnancy.
Very few women suffer from an incompetent cervix where there is no history of previous operation or pregnancy.
Fortunately an incompetent cervix is a comparatively rare condition. The diagnosis cannot be made until after the first miscarriage when the history alone is sufficiently definite and classical for the cause to be established. If there is any doubt a special X-ray of the uterus is diagnostic. The treatment usually recommended is the insertion of a special stitch (Shirodkar suture) around the cervix either before or during pregnancy. The preference in Great Britain is usually to insert the suture during pregnancy and this is done under anaesthetic, generally at the 14th week or shortly afterwards, at which time the pregnancy is known to be in its most stable condition.
The Shirodkar suture is made of nylon or a similar material which is placed around the cervix like a purse-string to prevent it from opening. It is a comparatively simple procedure. This treatment has a very high success rate and over 75 per cent of the pregnancies proceed to term. The stitch is removed at about the 38th week (or before, if the woman goes into labour at an earlier date) and the onset of natural labour may begin quite soon afterwards or not until term.