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What is an incompetent cervix?

What is an incompetent cervix?


A very rare complication that can happen during the second trimester of pregnancy involves the woman's cervix spontaneously and painlessly opening, usually very rapidly and unpredictably, with minimal or no uterine contractions.



This leads to a late miscarriage, if the pregnancy is between 12 to 20 weeks (as classified in Australia), or a very premature birth if the pregnancy is between 20 to 28 weeks (occasionally it can occur as late as the first few weeks of the third trimester, up to 32 weeks). The medical term for this complication is 'cervical incompetence' or the woman having an 'incompetent cervix' (not a very desirable description). It is thought that 20 - 25 per cent of babies born between 12 and 28 weeks do so because of this condition.

Why does cervical incompetence happen?

The cause of cervical incompetence is not fully understood, but it is believed to be due to a physical weakness in the woman's cervix that makes it susceptible to 'giving way' as the baby becomes larger and heavier, placing increased pressure on the cervix as the pregnancy progresses. Even though the 'condition' has been described as early as the 17th century, accurate diagnosis in any individual woman is difficult because there are no well-defined or strict criteria to definitely say that a woman has an 'incompetent cervix'. This poses a dilemma for caregivers because in most cases, the condition is only identified in retrospect, after one (or more) of the woman's babies has been born in this manner.

Some other health aspects that are thought to be associated with an increased likelihood for a woman's cervix to respond in this way during pregnancy include:

  • Exposure to the drug DES (diethylstilbestrol) as an unborn child. This drug was given to women during the 1930s to 1970s with the aim of preventing miscarriage, but is no longer prescribed to pregnant women because of its many side effects on the baby in later life. 
  • Trauma to the cervix. This may happen through an extensive cone biopsy operation, used to diagnose and/or treat changes in the cells of the cervix after an abnormal pap test. Another factor may be a woman who has had repeated operations that involve dilation of the cervix (as a guide more than five operations, although it may be significantly more than this, depending on the individual woman), or an operation that has forcefully opened the cervix.

Sometimes there is no obvious health reason why the condition is present.

Ultrasound examinations

In recent years, caregivers have used vaginal ultrasound examinations to try and 'predict' which women could be at increased risk of having cervical incompetence. It is believed that women with a naturally short cervix may be more likely to experience this complication (as well as possibly premature birth).

A woman is thought to be at increased risk if her cervix is dilated more than 2.5 centimetres when pregnant, and/or the length of her cervix has shortened to less than 20 millimetres. However, it is important to be aware that the length of each individual woman's cervix varies (even when not pregnant) and about eight per cent of all women naturally have a cervix that is only eight millimetres long, yet they can still carry their pregnancy to term. The use of ultrasound to measure the cervix is also discussed in relation to premature birth with twins.

Another sign looked for with the ultrasound is 'funnelling' of the cervix. This is where the internal portion of the cervix (closest to the baby) has begun to open, even before the external portion of the cervix (closest to the woman's vagina) can be seen (or felt) to open.

'Funnelling' is more commonly known as 'effacement' when it happens during early labour. The cervix must thin out before it dilates (or opens) to allow the baby to be born. However, when the cervix thins prematurely without painful contractions, this can be related to cervical incompetence.

NOTE: Be aware that funnelling does not always lead to the cervix opening spontaneously. This sign is only predictive for about 20 per cent of women.

Physical signs

In most cases, women experiencing a late miscarriage or early premature birth due to the cervix spontaneously opening will have very few (if any) physical signs. However, some women will notice one or more of the following signs as their cervix changes and opens:

  • An increased vaginal discharge.
  • A small amount of bleeding or 'spotting'.
  • Cramping or period-like pain.
  • A feeling of pressure or 'heaviness'.

NOTE: Be aware that these signs can be experienced by many pregnant women for other unknown reasons, yet their cervix remains closed and their pregnancy continues as normal until the baby is due. However, if you notice these signs, then you should contact your caregiver.

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