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Cervical length Screening

Cervical screening for preterm delivery

 A tiny premature baby in the neonatal intensive care unit


The average pregnancy lasts approximately 40 weeks from your last menstrual period. This is called “term.” 

But some babies try to come earlier. Premature labour can result in preterm birth. 


Sometimes babies are born before their organs are mature enough and they may need to be admitted to a newborn intensive care nursery for extra treatment. This occurs in about 6% of pregnancies and it is something we try to avoid.

The risk of spontaneous preterm birth increases as the length of the cervix (neck of the womb) decreases. 

The length of the cervix can be measured accurately with trans-vaginal ultrasound examination. 


We are often asked to assess the length of the cervix in women who are at high risk of preterm birth. Sometimes we have to do this on two or more occasions to see if there has been any change.

What is a normal cervical length?


The length of the cervix changes with advancing gestation.

At 20 weeks gestation the average cervical length is about 40mm but there is wide variation. 


If the length is less than 25 mm it indicates a higher risk of preterm birth.

If the length is less than 15 mm it is clearly abnormal. We will discuss the situation with your doctor or midwife.

Who may be asked to have cervical screening?

We screen all women who come for a 19-20 week scan, unless they do not want it. 

This is because of the evidence that about 45% of preterm births can be prevented if a short cervix is detected in time.


Women who are at higher risk of premature labour are often asked to have screening performed at their routine 18-20 week scan and later. 

This includes

  • women who have had a previous premature delivery
  • women who have had previous surgery on their cervix
  • women with a cervical suture
  • women who have had a painless second trimester fetal loss
  • women with multiple pregnancies may also benefit from cervical screening.