Cervical length is the length of the closed part of your cervix, measured in millimetres on a transvaginal scan. It shortens slowly through pregnancy: in the largest reference study the median fell from 43.3 mm at 16 weeks to 40 mm at 24 weeks and 30.3 mm at 36 weeks. Before 24 weeks, 25 mm or less counts as a short cervix.
This page has the normal range for every week from 16 to 36, a checker for the number on your scan report, the cut-offs NICE, ISUOG, SMFM and FIGO use, and what doctors offer when the cervix is short. It also explains funnelling, why the scan is done internally, and how cervical length differs from dilation and from fundal height.
The short answer: between 16 and 24 weeks, eight in ten women measure roughly 30 to 50 mm; at 20 weeks the median is 41.9 mm. A transvaginal measurement of 25 mm or less before 24 weeks is called a short cervix and raises the chance of preterm birth. Vaginal progesterone, and for some women a stitch, lowers that chance.
Normal cervical length by week
The most detailed week-by-week reference comes from Salomon and colleagues in Paris, who measured the cervix once in each of 6,614 singleton pregnancies between 16 and 36 weeks. They found that cervical length is not spread evenly around an average, so they built centiles with a method that allows for the skew. The table gives the median for each week, the low centiles that matter for a short cervix, the 90th centile, and the share of women who measured under 25 mm.
| Week | 5th centile, mm | 10th centile, mm | Median, mm | 90th centile, mm | Under 25 mm, % |
|---|---|---|---|---|---|
| Week 16 | 32.2 | 34.9 | 43.3 | 50.9 | 0.5 |
| Week 17 | 31.5 | 34.2 | 43 | 50.8 | 0.7 |
| Week 18 | 30.8 | 33.6 | 42.6 | 50.8 | 0.9 |
| Week 19 | 30.1 | 33 | 42.3 | 50.7 | 1.2 |
| Week 20 | 29.3 | 32.3 | 41.9 | 50.5 | 1.6 |
| Week 21 | 28.5 | 31.6 | 41.5 | 50.3 | 2.1 |
| Week 22 | 27.7 | 30.9 | 41 | 50.1 | 2.7 |
| Week 23 | 26.8 | 30.1 | 40.6 | 49.9 | 3.4 |
| Week 24 | 25.8 | 29.2 | 40 | 49.6 | 4.2 |
| Week 25 | 24.7 | 28.3 | 39.4 | 49.1 | 5.3 |
| Week 26 | 23.5 | 27.2 | 38.6 | 48.5 | 6.6 |
| Week 27 | 22.4 | 26.2 | 37.8 | 47.9 | 8.1 |
| Week 28 | 21.4 | 25.2 | 37.1 | 47.3 | 9.6 |
| Week 29 | 20.4 | 24.3 | 36.3 | 46.8 | 11.2 |
| Week 30 | 19.5 | 23.5 | 35.6 | 46.3 | 12.9 |
| Week 31 | 18.6 | 22.6 | 34.9 | 45.8 | 14.7 |
| Week 32 | 17.8 | 21.8 | 34.2 | 45.2 | 16.8 |
| Week 33 | 17 | 20.9 | 33.3 | 44.5 | 19.1 |
| Week 34 | 16.1 | 19.9 | 32.3 | 43.7 | 22 |
| Week 35 | 15.2 | 19 | 31.3 | 42.8 | 25.1 |
| Week 36 | 14.4 | 18.1 | 30.3 | 41.8 | 28.6 |
Source: Salomon LJ et al., Ultrasound in Obstetrics & Gynecology 2009, Table 2; 6,614 transvaginal measurements. Every centile was checked against the study's own formula. No row is given before 16 or after 36 weeks because the study did not measure there.
Read down the last column. At 16 weeks only 0.5% of women measured under 25 mm; at 24 weeks it was 4.2%, at 32 weeks 16.8% and at 36 weeks 28.6%. The same 25 mm reading is rare and worrying at 18 weeks and fairly common at 34. That is why guidelines apply the 25 mm line to scans before 24 weeks.
Other populations land a little lower. In a large American study, Iams and colleagues measured 2,915 women at about 24 weeks and found a mean of 35.2 mm, with the 10th percentile at 26 mm. Women at or below that 10th percentile were 6.2 times as likely to give birth spontaneously before 35 weeks. The risk rose step by step as the cervix got shorter, with no sudden cliff at one number.
Cervical length checker
Enter the week of your scan and the cervical length from the report. The checker places it against the Salomon centiles for that week and against the 25 mm line. It does not diagnose anything: your doctor or midwife reads the number with your history and any symptoms.
—
Whatever the number: regular tightenings, period-like pain, fluid or bleeding before 37 weeks need a call to your maternity unit straight away.
What counts as a short cervix: the guideline cut-offs
The main guidelines agree on 25 mm and differ on what to do between 20 and 25 mm. Every one of them means a transvaginal measurement in a singleton pregnancy.
| Guideline | Cut-off | What it recommends |
|---|---|---|
| NICE NG25 (UK, updated 2022) | 25 mm or less on a scan at 16+0 to 24+0 weeks | With a previous preterm birth or a loss from 16 weeks: a choice of vaginal progesterone or a stitch. With either the history or the short cervix alone: consider progesterone, continued until at least 34 weeks. |
| ISUOG (2022) | 25 mm or less before 24 weeks | Transvaginal screening for all women between 18 and 24 weeks; vaginal progesterone (for example 200 mg every night) from diagnosis until 36 weeks; pessaries only in research. |
| SMFM Consult #70 (US, 2024) | 25 mm or less is short; 20 mm or less before 24 weeks | For women without a previous preterm birth: progesterone recommended at 20 mm or less, considered at 21–25 mm; no stitch unless the cervix is opening; no pessary. |
| FIGO (2021) | Under 25 mm for a stitch | Vaginal progesterone for a singleton pregnancy with a previous preterm birth or a short cervix; a scan-indicated stitch below 25 mm after a previous preterm birth or mid-pregnancy loss. |
NICE also uses cervical length later, in a different situation. If you have symptoms of preterm labour from 30 weeks, a scan showing more than 15 mm means labour is unlikely; 15 mm or less means you are treated as being in preterm labour. Our guide to the signs of preterm labour covers that hospital assessment.
How cervical length is measured
The scan is done with a slim probe placed in the vagina, with your bladder empty. It takes a few minutes. The sonographer finds the internal os (the inner opening, towards the baby) and the external os (the outer opening, towards the vagina), enlarges the image so the cervix fills half to three quarters of the screen, and measures the closed canal between them. ISUOG asks for at least three measurements and reports the shortest technically correct one.
A transabdominal scan, through the bump, is not used for this. A full bladder and pressure on the probe both make the cervix look longer than it is, and ISUOG notes that the transabdominal equivalent of 25 mm would be around 36 mm. SMFM recommends that every measurement used to guide treatment is transvaginal.
What the report means
- Cervical length, CL, 38 mm: the closed part of the canal.
- Internal os closed: no funnel at the top of the canal.
- Funnelling (sometimes graded as Y, V or U shapes): the membranes have pushed into the upper canal. Salomon saw it in 6.5% of scans. ISUOG says no funnel measurement predicts preterm birth on its own, so the funnel is never added to the length.
- Sludge: debris in the fluid just above the cervix, which some reports mention. Your doctor will say whether it changes anything for you.
When cervical length is measured
Practice varies by country. ISUOG recommends a transvaginal measurement for every pregnant woman between 18 and 24 weeks, ideally at the anomaly scan. SMFM in the US says universal screening cannot be mandated but is reasonable. Where screening is not routine, the scan is offered to women at higher risk: SMFM recommends measuring every 1 to 2 weeks from 16 to 24 weeks after a previous spontaneous preterm birth, and NICE also ties its advice to a previous loss from 16 weeks, waters breaking early in a previous pregnancy, or surgery on the cervix.
No guideline sets a target cervical length for the weeks just before birth. Salomon's data stop at 36 weeks with a median of 30.3 mm. After that, the cervix softens and shortens as the body prepares for labour, and midwives and doctors judge readiness for birth by examination rather than by a scan number.
What happens if your cervix is short
A short cervix is a risk marker, not a diagnosis of preterm labour. The options depend on the length, the week and your history.
Vaginal progesterone
Progesterone pessaries or gel, used every night, are the first-line treatment for a short cervix in a singleton pregnancy. In a pooled analysis of five trials and 974 women with a cervix of 25 mm or less, Romero and colleagues found that vaginal progesterone cut birth before 33 weeks by more than a third (relative risk 0.6). The larger EPPPIC analysis of individual women's data (9 trials, 3,769 women at high risk) found a relative risk of 0.8 for birth before 34 weeks, with the biggest absolute benefit when the cervix was short. NICE continues treatment until at least 34 weeks, ISUOG until 36.
Weekly 17-OHPC injections are no longer an option in the US: the FDA withdrew them in 2023, and SMFM advises against prescribing them.
Cervical stitch (cerclage)
A cerclage is a stitch placed around the cervix through the vagina. It helps most when a short cervix comes on top of a previous preterm birth or mid-pregnancy loss: NICE and FIGO both offer it then. A Cochrane review of 15 trials (3,490 women) found that a stitch in high-risk singleton pregnancies lowered birth before 34 weeks (relative risk 0.8). For a short cervix with no such history, SMFM advises progesterone and no stitch unless the cervix has started to open.
Cervical pessary
A pessary is a silicone ring placed around the cervix. The US TOPS trial in women with a short cervix was stopped early: the pessary did not reduce preterm birth, and fetal or newborn deaths were higher with it (13.3% against 6.8%). SMFM now recommends against a pessary for a singleton pregnancy with a short cervix, and ISUOG limits it to research.
Bed rest is not on the list. SMFM recommends against activity restriction, and it brings its own risks, such as blood clots.
Cervical length is not fundal height
The two get mixed up because both are lengths measured in pregnancy. Fundal height is measured on the outside, with a tape from your pubic bone to the top of your womb, in centimetres, to follow the baby's growth. Cervical length is measured on the inside, on a transvaginal scan, in millimetres, and says nothing about growth: it estimates the chance of an early birth. A small bump does not mean a short cervix, and the reverse holds too.
When to call your maternity unit the same day
Call your midwife or maternity unit straight away, even at night, if before 37 weeks you have: regular tightenings or period-like cramps, a new low backache that comes and goes, pressure low in your pelvis as if the baby is pushing down, fluid leaking or gushing from your vagina, vaginal bleeding, or a change in discharge to watery, mucus-like or bloody. Do not wait for your next scan, whatever your last cervical length was. Call 999 or 911, or your local emergency number, for heavy bleeding or if you collapse.
A number on a report is never on this list by itself. A short cervix with no symptoms is something to plan around with your doctor or midwife at the appointment, usually the same week.
Frequently Asked Questions
What is a normal cervical length at 20 weeks?
In Salomon's reference study of 6,614 pregnancies, the median cervical length at 20 weeks was 41.9 mm, with the 10th centile at 32.3 mm and the 5th at 29.3 mm. Only 1.6% of women measured under 25 mm at this week, which is why 25 mm is the line most guidelines use before 24 weeks.
What cervical length is considered short?
A transvaginal measurement of 25 mm or less before 24 weeks is called a short cervix by NICE, ISUOG and the Society for Maternal-Fetal Medicine (SMFM). SMFM recommends vaginal progesterone at 20 mm or less and a shared decision between 21 and 25 mm. After 24 weeks the cervix shortens on its own, so the same number carries less weight.
Is cervical length checked at the anatomy scan?
It depends on where you live. ISUOG recommends measuring every pregnant woman's cervix transvaginally at the second-trimester scan, between 18 and 24 weeks, when feasible. SMFM calls universal screening reasonable but not mandatory. Where screening is not routine, women with a previous preterm birth are scanned instead; SMFM advises a measurement every 1 to 2 weeks from 16 to 24 weeks.
Can cervical length be measured on an abdominal ultrasound?
It can be seen, but the number is not used to decide treatment. ISUOG and SMFM both say measurements that guide care should be transvaginal. Through the abdomen, a full bladder pushes the cervix longer, and ISUOG notes that a transabdominal cut-off would need to be about 36 mm to match 25 mm on a transvaginal scan.
Can cervical length increase during pregnancy?
The trend through pregnancy is downwards: in Salomon's data the median fell from 43.3 mm at 16 weeks to 30.3 mm at 36 weeks. Two scans can still differ by a few millimetres. ISUOG warns that a full bladder or too much pressure from the probe makes the cervix look longer, which is why the shortest of three careful measurements is the one reported.
What cervical length is needed for a cerclage?
Cerclage depends on your history as well as the number. NICE offers a choice of a stitch or vaginal progesterone when a scan at 16 to 24 weeks shows 25 mm or less and you have had a preterm birth or a loss from 16 weeks. FIGO advises a stitch below 25 mm in the same situation. Without that history, SMFM recommends against a stitch for a short cervix unless it has started to open.
What does funnelling mean on a cervical length scan?
Funnelling means the membranes around the baby have pushed into the top of the cervical canal, so the canal looks like a funnel or a V or U shape on the scan. ISUOG says no measurement of funnelling has been shown to predict preterm birth on its own, so only the closed part of the cervix is counted as its length.
Is cervical length the same as dilation or effacement?
No. Cervical length is measured on a transvaginal scan, in millimetres, usually in mid-pregnancy, and the cervix is closed. Effacement is the cervix thinning and shortening as labour approaches, often described in percent, and dilation is how far it has opened, in centimetres, checked by an internal examination in labour.
Cervical length by week — at a glance
Cervical length is measured on a transvaginal scan, in millimetres, and shortens slowly through pregnancy: in Salomon's study of 6,614 pregnancies the median fell from 43.3 mm at 16 weeks to 40 mm at 24 and 30.3 mm at 36. NICE, ISUOG and SMFM call 25 mm or less before 24 weeks a short cervix. Vaginal progesterone lowers the chance of early birth; a stitch helps most after a previous preterm birth; pessaries are not recommended. Regular tightenings, fluid or bleeding before 37 weeks need a call to your maternity unit straight away.
Inside Baby Novum: the ultrasound tracker keeps each scan with its date, week, the doctor's comment and a photo of the report, so every cervical length measurement is on your phone at the next appointment. One record is free; Premium keeps every scan.
Read next
Sources
- Salomon et al., Ultrasound in Obstetrics & Gynecology 2009 — Reference range for cervical length throughout pregnancy (Table 2)
- Iams et al., New England Journal of Medicine 1996 — The length of the cervix and the risk of spontaneous premature delivery
- NICE NG25 — Preterm labour and birth (2015, updated 2022)
- ISUOG Practice Guidelines (2022) — Role of ultrasound in the prediction of spontaneous preterm birth
- Society for Maternal-Fetal Medicine Consult Series #70 (2024) — Management of short cervix in individuals without a history of spontaneous preterm birth
- Society for Maternal-Fetal Medicine (2016) — The role of routine cervical length screening in selected high- and low-risk women
- FIGO good practice recommendations on progestogens for prevention of preterm delivery (2021)
- FIGO good practice recommendations on cervical cerclage for prevention of preterm birth (2021)
- Society for Maternal-Fetal Medicine statement (2023) — Response to the FDA withdrawal of 17-OHPC
- EPPPIC Group, The Lancet 2021 — Progestogens for preventing preterm birth: meta-analysis of individual participant data
- Romero et al., American Journal of Obstetrics & Gynecology 2018 — Vaginal progesterone in singleton pregnancies with a short cervix
- Hoffman et al., JAMA 2023 — Cervical pessary for prevention of preterm birth (TOPS trial)
- Alfirevic et al., Cochrane Database of Systematic Reviews 2017 — Cervical stitch (cerclage) for preventing preterm birth
Baby Novum is an informational and tracking tool, not a medical device, and does not provide medical diagnosis or treatment. Always consult your doctor or midwife about your pregnancy.