Cervical length scans and the fetal fibronectin test

If you’re at higher risk of premature birth, you may be offered a scan to measure your cervix, or a swab test. Here’s who gets them, what happens and what the results mean.

Why are they offered?

Most pregnancies go to full term. But some people are at higher risk of going into labour early. The NHS lists risk factors including giving birth before 34 weeks in a previous pregnancy, a miscarriage from 16 weeks, waters breaking early in a previous pregnancy, a past injury to the cervix (for example from surgery), or carrying twins or more. Being at higher risk does not mean you will have a premature baby.

Cervical length scan

The NHS says a vaginal (internal) scan between 16 and 24 weeks can measure the length of your cervix. A cervix shorter than 25mm is linked to a higher risk of early labour and birth. One hospital leaflet explains you may need only one scan, or the scan may be repeated every 2 to 4 weeks between 16 and 24 weeks. You’re asked to empty your bladder first, and the probe is gently placed in the vagina.

If your cervix is short

The NHS lists treatments that may be offered, usually starting before 24 weeks:

Progesterone. A hormone medicine inserted into the vagina. The leaflet says it’s usually started between 16 and 24 weeks and continued until at least 34 weeks.
A cervical stitch (cerclage). A stitch placed in the cervix under anaesthetic, which the leaflet says can be inserted up to 24 weeks.

The leaflet also says not all preterm births can be prevented, and your care plan is based on the length of your cervix and your own history.

Fetal fibronectin (fFN) test

Fetal fibronectin is a protein that acts like glue holding the amniotic sac to the womb lining. When birth is getting near, it can leak into the vagina. Tommy’s describes a test, done with a vaginal swab, used for people with symptoms of premature labour. Results can come back in under ten minutes. Tommy’s research page (the QUIDS study) describes a newer version of the test that measures how much fFN is present, combined with other risk factors, to predict whether labour is likely within 7 days. The page does not say whether it is routinely used on the NHS, so ask your maternity unit what they use.

Signs of premature labour

If you’re under 37 weeks, the NHS says to call your midwife or maternity unit straight away if you have regular contractions or tightenings, period-type pains, a gush or trickle of fluid, or unusual backache. Staff will check whether you’re in labour and what is happening. If you’re ever unsure or worried, call. See Braxton Hicks and signs of labour.

Tommy’s midwives are on 0800 0147 800 if you want to talk anything through.

Questions people ask

Does a short cervix mean I’ll have a premature baby?

Not necessarily. The NHS links a shorter cervix with a higher risk, and the hospital leaflet says having risk factors does not necessarily mean you will have a premature birth. Treatment may be offered.

Is the scan uncomfortable?

It is an internal scan, so tell the person scanning if you’re uncomfortable. They can stop.

What if I have no risk factors?

Your midwife can talk through your own risk and whether a scan is right for you.

What if I think I’m in early labour?

Call your maternity unit straight away, day or night.