Cervical length scan and preterm birth risk

Medically written by Dr. Sushma Dhakal, MBBS · MD (Obstetrics & Gynaecology) · Fellowship in Fetal Medicine · Updated 30 October 2026 · 3 min read

The cervix is the neck of the womb. During pregnancy it normally stays long and closed until near term. If it shortens early, the chance of preterm birth (before 37 weeks) increases. A cervical length scan measures it, so that women at higher risk can be offered treatment in time.

How is it done?

The most accurate measurement is with a transvaginal scan. A slim probe is placed gently in the vagina, the bladder is emptied first, and the scan takes about 5–10 minutes. It is safe, does not affect the pregnancy, and is usually only mildly uncomfortable. An abdominal scan is less reliable for this measurement.

When and for whom?

Usually between 16 and 24 weeks, often at the time of the anomaly scan (18–22 weeks).
Women with a previous preterm birth or late miscarriage (after 14–16 weeks) may have serial scans, every 2–4 weeks from about 16 weeks.
Previous cervical surgery (such as a cone biopsy or LLETZ), a uterine abnormality, or twins may also be reasons for assessment.

What do the results mean?

A cervical length above about 25 mm before 24 weeks is generally reassuring.
A cervix of 25 mm or less is called a short cervix. The shorter it is, the higher the chance of preterm birth, but many women with a short cervix still carry their baby to term, especially with treatment.
The scan may also show "funnelling", where the inner part of the cervix is opening.

What can be done?

Vaginal progesterone: in women with a short cervix and a single baby, nightly vaginal progesterone has been shown to reduce the risk of preterm birth. It is usually continued until about 34–36 weeks.
Cervical cerclage (stitch): a stitch placed around the cervix may be offered to women with a short cervix and a history of preterm birth or late miscarriage, or with certain cervical problems.
Planning ahead: if preterm birth seems likely, steroid injections to help the baby's lungs and magnesium sulphate for brain protection may be given at the right time, and delivery is planned at a hospital with neonatal intensive care.

Treatment for twins with a short cervix is less clear, and options will be discussed individually.

Warning signs of preterm labour

Contact your doctor or hospital promptly if you have regular tightenings or period-like pain, low backache that comes and goes, pelvic pressure, a change in vaginal discharge, bleeding, or leaking fluid before 37 weeks. Early assessment allows the right care to start quickly.

Have questions about your pregnancy?

Book a consultation with Dr. Sushma Dhakal, Fellowship-trained fetal medicine specialist (FMF UK certified).

Book this scan Request Counselling

Frequently asked questions

What cervical length is considered short?

25 mm or less before 24 weeks, measured by transvaginal scan.

Is a transvaginal cervical scan safe in pregnancy?

Yes. It is gentle, quick and does not affect the pregnancy.

Does a short cervix mean my baby will come early?

Not necessarily. It increases the chance, but treatment such as progesterone or a stitch can lower the risk, and many women still deliver at term.

When should cervical length be checked?

Usually between 16 and 24 weeks, often at the anomaly scan; earlier and repeatedly for women with a previous preterm birth.

This guide is for general information only and is not a substitute for personal medical advice. Every pregnancy is different. Please consult your doctor about your own situation. In an emergency, go to the nearest hospital.