Why might you need a cervical length scan in pregnancy?

Your cervix is a tube of muscle linking your uterus (womb) with your vagina. During pregnancy a plug of mucus forms at the top of the cervix efficiently sealing off the womb and helping to keep your baby safe from infection. Sometimes during pregnancy your cervix can shorten abnormally and open too soon, which puts your baby at risk.

This article explains what can happen to your cervix during pregnancy, what a Cervical Length Scan is for and why you might need a Cervical Length Scan.

When you're not pregnant your cervix is rigid and tight, but it will slowly soften, shorten and dilate as your progress through pregnancy. The cervix should stay closed throughout pregnancy; then when labour is about to begin the plug of mucus will come away (this is called a show) and contractions to dilate the cervix and push your baby out will begin shortly afterwards, usually within hours or a few days.

What can go wrong in pregnancy?

If you've had previous late miscarriages or premature births, had surgery to your cervix because of a previous abnormal smear or you are pregnant with twins or more, you may be at higher risk of your cervix shortening abnormally and opening too soon, putting your baby at risk.

If there are any concerns about your cervix opening up or shortening during your pregnancy you may be advised to have a Cervical Length Scan to assess whether you are at higher risk of late miscarriage or premature labour. This is because there’s an increased risk of you developing an infection. It isn't entirely clear whether the infection causes the cervix to loosen and shorten or whether the cervix weakens allowing an infection to get into the uterus.

If your cervix is shorter than normal or incomplete, you are at higher risk of miscarriage or premature labour and you may need a series of further monitoring scans/treatment to prevent this happening.

What does the scan measure?

A Cervical Length Scan measures the length of the cervix and is performed internally by placing an ultrasound probe into the vagina next to the cervix whilst you have an empty bladder.

Your sonographer can then get a clear view of your cervix and check to see if the length is in the normal range for your stage of pregnancy. They will look for any signs of an incomplete cervix where it has begun to dilate (open) and shorten.

What will happen if my cervix is too short?

If your scan reveals that your cervix is too short for your dates and is beginning to open, your obstetrician may consider further scans later in your pregnancy to check it's not opening too fast, or if you require surgical treatment.

Sometimes your obstetrician will decide to insert a cervical stitch or suture around the cervix opening into the womb; usually between 12 and 24 weeks of pregnancy.

The Royal College of Obstetricians and Gynaecologists says a stitch helps keep the cervix long and closed and reduces the risk of late miscarriage or premature birth.

The RCOG recommends cervical length scans may be offered to women who've had one or two late miscarriages/premature labours before 34 weeks and states if the cervix measure less than 25mm they may be advised to have a cervical suture. The College also advises that if a woman has had three or more late miscarriages/premature births she may be advised to have a cervical suture at 12 to 14 weeks. The stitch will normally be removed at 36/37 weeks unless the woman goes into labour first.

What happens when a cervical stitch is not appropriate?

Cervical sutures are not recommended by the RCOG if a woman is:

  • More than 24 weeks pregnant

  • Carrying twins or triplets

  • Your womb is an abnormal shape or

  • You've had previous treatment to the cervix for an abnormal smear.

Cervical stitches also can't be done if you're already in labour or your waters have broken, you have signs of an infection or there are concerns about your baby's health.

Women who are not suitable for a cervical suture may be offered follow-up monitoring scans to check the cervix isn’t opening too quickly until 24 weeks and then can be prescribed corticosteroid treatments from 23 weeks to strengthen their baby's lungs in the event that they are born prematurely.

Your preferred healthcare professional will work with you to make sure your baby is safe and well throughout your pregnancy, using the report you get from your ultrasound scan to make the best decisions with you.