Cervical Length Assessment
The cervical length scan is a targeted ultrasound that measures the length of the cervix and assesses it for any changes that may indicate an increased risk of preterm birth. It is a straightforward, well-established part of pregnancy monitoring — although it is currently routinely offered to all pregnant women, the cervical length scan is especially important for women with increased risk of preterm birth.
What Is a Cervical Length Scan?
The cervix is the lower part of the uterus that connects to the vagina. During pregnancy, it remains long and closed, acting as a protective barrier for the baby. As the pregnancy approaches full term, the cervix gradually shortens and opens in preparation for labour. When this process begins too early, it can increase the risk of preterm birth.
A cervical length scan measures the length of the cervix using ultrasound, and looks for any signs of premature change — most importantly shortening or funneling, which is when the cervix begins to open from the inside (at the internal os) before it should.
The scan can be performed either transabdominally (over the abdomen) or transvaginally (internally). If the cervix can be clearly visualized and appears reassuringly long on the transabdominal view, that may be sufficient. However, the transvaginal approach provides a more accurate measurement and is generally preferred — particularly when the transabdominal view is suboptimal or the cervix appears short. The procedure is the same as other transvaginal scans: a slim, covered probe is gently inserted into the vagina. It should not be painful, though some women find it mildly uncomfortable.
What Does the Scan Look For?
The main findings assessed are:
1. Cervical length: A cervical length of less than 25 mm (2.5 cm) is considered short and is associated with a significantly increased risk of preterm birth.
2. Funneling: This refers to premature opening of the cervix at the internal os — the upper end — while the external os (lower end) remains closed. It indicates that the cervix is beginning to yield under pressure from above, and is a sign of cervical insufficiency.
3. Amniotic sludge: In some cases, particulate material may be visible in the amniotic fluid near the internal os. This finding, known as sludge, may indicate subclinical infection or inflammation and has been associated with an increased risk of preterm birth.
When Is a Cervical Length Scan Performed?
For low-risk pregnancies (singleton pregnancy, no prior preterm birth): cervical length is routinely assessed once, at the time of the mid-trimester fetal anatomy scan between 18 and 22 weeks. This is already part of what your doctor checks during that scan — you can read more about what else is assessed at that visit on our Fetal Anatomy Scan page.
For higher-risk pregnancies (previous preterm birth or other risk factors): serial transvaginal cervical length measurements are performed every 2 weeks, starting from 16 weeks until 24 weeks. Serial monitoring is important because the cervix can appear adequate at one visit and show progressive shortening at the next — a pattern that can guide timely intervention.
How Does Cervical Length Relate to Preterm Birth Risk?
In the general population of women carrying a single baby with no prior preterm birth, the background risk of delivering before 37 weeks is approximately 10%. Several factors can increase this risk, including a history of preterm birth in a previous pregnancy, a twin or higher-order pregnancy, certain uterine abnormalities, infection, and lifestyle factors such as smoking.
Cervical length provides an objective, measurable way to stratify this risk further. A cervical length of less than 25 mm in the mid-trimester increases the risk of delivering before 35 weeks by approximately 6.5 times compared to women with a longer cervix. This is a clinically significant difference and forms the basis for targeted intervention.
What Happens If a Short Cervix Is Found?
If a short cervix is identified, your doctor will discuss management options based on your specific situation — including your cervical length measurement, gestational age, and overall history.
Vaginal progesterone.
For women with a short cervix on ultrasound but no prior preterm birth, vaginal progesterone is the first-line treatment. It is administered as a suppository inserted into the vagina, usually daily. Progesterone delivered locally helps support cervical integrity and has been shown to reduce the risk of preterm birth in women with a short cervix. It is generally well tolerated, though it may cause some increase in vaginal discharge.
Serial monitoring.
Once a short cervix is identified, transvaginal cervical length scans are repeated every 1 to 2 weeks to monitor for further shortening or the development of funneling.
Cervical cerclage.
If the cervical length falls below approximately 10 mm, or if funneling develops, a cervical cerclage may be recommended. This is a procedure performed under anaesthesia in which a suture is placed around the cervix to keep it closed. It is most effective when performed before significant dilation has occurred, and has good evidence supporting its use in appropriately selected patients.
Both vaginal progesterone and cerclage, when used in the right circumstances, have been shown to meaningfully reduce the risk of preterm birth.
Should I Ask for a Cervical Length Scan?
If you are having your mid-trimester anatomy scan, cervical length assessment is typically included as part of that appointment — so it is likely already on your doctor’s checklist.
If you have had a previous preterm birth, late miscarriage, or cervical procedure (such as a LEEP/cone biopsy), it is worth specifically discussing serial cervical length monitoring with your doctor.
If you have any concerns or questions about your preterm birth risk, please feel free to book a consultation.
References
1. Berghella V, Baxter JK, Hendrix NW. Cervical assessment by ultrasound for preventing preterm delivery. Cochrane Database Syst Rev. 2013;(1):CD007235. doi:10.1002/14651858.CD007235.pub3.
2. American College of Obstetricians and Gynecologists. Prediction and Prevention of Preterm Birth: ACOG Practice Bulletin No. 234. Obstet Gynecol. 2021;138(2):e65-e90. (Reaffirmed 2023).
3. Romero R, Nicolaides KH, Conde-Agudelo A, et al. Vaginal progesterone decreases preterm birth ≤ 34 weeks of gestation in women with a singleton pregnancy and a short cervix: an updated meta-analysis. Ultrasound Obstet Gynecol. 2016;48(3):308-317. doi:10.1002/uog.15953.
4. Cunningham FG, Leveno KJ, Dashe JS, Hoffman BL, Spong CY, Casey BM. Williams Obstetrics. 26th ed. New York: McGraw-Hill; 2022.
5. Iams JD, Goldenberg RL, Meis PJ, et al. The length of the cervix and the risk of spontaneous premature delivery. N Engl J Med. 1996;334(9):567-572. doi:10.1056/NEJM199602293340904.
Maternal–Fetal Medicine Specialist in Bangkok
Dr. Narisra Srikureja Firer (Dr. Niti)
Dr. Niti is a Bangkok-based OBGYN with subspecialty training in Maternal–Fetal Medicine (MFM). She provides comprehensive care for women across all stages of life, including pregnancy care, deliveries, and general gynecological conditions.
She dedicates her time to private practice at Ruamjairak Hospital and Mission Hospital, and serves as a Maternal–Fetal Medicine Consultant at a large government tertiary care center.
To read Dr. Niti’s full bio, click here.
