Short Cervix Without Preterm Birth: SMFM 2024 Guidelines
Clinical Context
In 2024, the Society for Maternal-Fetal Medicine published Consult Series #70 on managing a short cervix in pregnant individuals without a history of spontaneous preterm birth. This document is crucial for a common scenario: during screening or additional cervicometry, a cervical length of 25 mm or less is detected, the pregnancy is singleton, there are no complaints, and there is no history of preterm birth.
The key logic of SMFM: the diagnosis is made only with correctly performed transvaginal cervicometry; the threshold for diagnosis is ≤25 mm; active medical prevention is most justified at ≤20 mm before 24 weeks. Mechanical interventions without additional indications should not become an automatic response to a short cervix.
Who the Algorithm Applies To
The recommendations apply to patients with a singleton pregnancy, no history of spontaneous preterm birth, and asymptomatic cervical shortening in the second trimester. This is crucial: the strategy for patients with a history of preterm birth, clinical contractions, premature rupture of membranes, or obvious cervical dilation may differ and should not be mechanically transferred from this algorithm.
Before making a decision, three conditions should be confirmed: the pregnancy is singleton; the measurement is performed transvaginally and standardized; the patient does not have a clinical picture of preterm labor. If at least one condition is not met, the diagnosis and clinical scenario should be clarified first.
Diagnosis: Only Transvaginal Cervicometry
SMFM recommends performing all cervical length measurements used for therapeutic decision-making transvaginally and using a standardized method. Transabdominal assessment may be useful as an approximate part of an ultrasound examination, but it should not replace transvaginal measurement if the result affects the prescription of progesterone, discussion of cerclage, or pessary.
The diagnostic threshold for a short cervix in this group is ≤25 mm in the second trimester. Thus, the value of 25 mm is not considered 'borderline normal' for the purposes of this consensus: it falls within the definition of a short cervix. Meanwhile, the therapeutic threshold for confidently prescribing progesterone is lower — ≤20 mm.
Threshold Values and Decisions
| Transvaginal Cervical Length | Interpretation by SMFM 2024 | Strategy |
|---|---|---|
| ≤25 mm | Short cervix in the second trimester in a patient without a history of spontaneous preterm birth | Assess gestational age, symptoms, singleton status; proceed to treatment stratification |
| ≤20 mm before 24 weeks | Group with the most clear indication for prevention | Prescribe vaginal progesterone to reduce the risk of preterm birth |
| 21–25 mm before 24 weeks | Short cervix, but with less evidential certainty than at ≤20 mm | Consider vaginal progesterone after discussing risks, benefits, and preferences |
| 10–25 mm without dilation | Sonographically short cervix without clinical dilation | Do not perform cerclage based solely on such cervicometry |
Vaginal Progesterone
According to SMFM Consult Series #70, vaginal progesterone is recommended for asymptomatic patients with a singleton pregnancy and a transvaginal cervical length of ≤20 mm if the diagnosis is made before 24 weeks. The goal of prescribing is to reduce the risk of preterm birth.
For a cervical length of 21–25 mm, SMFM recommends not automatic prescription but discussion with the patient. For the physician, this means: a result of 22, 23, 24, or 25 mm is not ignored, but the decision on progesterone is made individually. The discussion includes gestational age, measurement reliability, associated risk factors, and the patient's willingness to undergo therapy.
SMFM specifically notes that 17-alpha-hydroxyprogesterone caproate should not be prescribed for the treatment of a short cervix. Thus, in this scenario, the focus is on vaginal progesterone, not replacing it with the injectable form of 17-OHPC.
Cerclage: When Not Needed
One of the practical emphases of the document is limiting unwarranted cerclage. For patients without a history of spontaneous preterm birth, with a sonographic cervical length of 10–25 mm, and without cervical dilation, SMFM recommends not performing cerclage. This specifically applies to situations where the only basis is ultrasound shortening.
This approach helps avoid an invasive procedure without proven benefit in this group. If dilation is detected upon examination, the clinical scenario is different: the decision should not be made based on the table for asymptomatic sonographically short cervix without dilation.
Pessary
SMFM recommends not placing a cervical pessary for the prevention of preterm birth in patients with a singleton pregnancy and a short cervix. The practical conclusion is straightforward: a pessary is not an alternative to vaginal progesterone for a cervix ≤20 mm and is not a standard 'enhancement' strategy for 21–25 mm.
If a pessary is considered locally as part of a research protocol or individual decision, it should be separated from the routine evidence-based recommendation of SMFM. In regular clinical practice, a short cervix alone is not an indication for a pessary.
Twin and Multiple Pregnancies
For twins, SMFM separately recommends not routinely using progesterone, pessary, or cerclage for the treatment of a short cervix outside of clinical research. Therefore, the algorithm for singleton pregnancies cannot be automatically transferred to multiple pregnancies. If a short cervix is detected in twins, it does not mean that the same vaginal progesterone recommended for singleton pregnancies with a cervix ≤20 mm should be prescribed.
SMFM Recommendation Classes
| Question | SMFM 2024 Recommendation | Class |
|---|---|---|
| Measurement Method | Use the transvaginal approach and standardized method | GRADE 1C |
| Diagnosis of Short Cervix | Define as ≤25 mm in the second trimester | GRADE 1C |
| Vaginal Progesterone at ≤20 mm before 24 weeks | Prescribe to reduce the risk of preterm birth | GRADE 1A |
| Vaginal Progesterone at 21–25 mm | Consider based on shared decision-making | GRADE 1B |
| 17-OHPC | Do not prescribe for the treatment of short cervix | GRADE 1B |
| Cerclage at 10–25 mm without dilation | Do not perform in the absence of dilation | GRADE 1B |
| Pessary in Singleton Pregnancy | Do not place for the prevention of preterm birth | GRADE 1B |
Practical Algorithm for Ultrasound Conclusion and Consultation
- Record that the cervical length was measured transvaginally; specify the numerical value in millimeters.
- If the length is ≤25 mm, indicate that this corresponds to a short cervix according to SMFM 2024 for this clinical group.
- At ≤20 mm before 24 weeks, recommend clinical consideration of vaginal progesterone.
- At 21–25 mm before 24 weeks, indicate that progesterone may be discussed individually.
- Do not propose cerclage solely due to a length of 10–25 mm if there is no cervical dilation.
- Do not recommend a pessary as standard prevention of preterm birth in singleton pregnancies.
The optimal wording in the ultrasound protocol should separate the fact of measurement from the therapeutic decision. For example: 'Transvaginal cervical length 20 mm; according to SMFM 2024 corresponds to a short cervix; in singleton pregnancy without a history of spontaneous preterm birth before 24 weeks, consideration of vaginal progesterone is recommended.' This entry helps the obstetrician-gynecologist quickly align the result with the current consensus.
Frequently asked questions
Is a cervical length of 25 mm without a history of preterm birth normal or short?
According to SMFM 2024, a transvaginal cervical length of ≤25 mm in the second trimester is considered a short cervix in this clinical group. However, mandatory prescription of vaginal progesterone is formulated for ≤20 mm before 24 weeks; at 21–25 mm, the decision is discussed individually.
Is cerclage needed for a cervical length of 18–20 mm if there is no history of preterm birth?
If the pregnancy is singleton, the patient is asymptomatic, and there is no cervical dilation, SMFM recommends not performing cerclage for a sonographic cervical length of 10–25 mm. The main recommended measure at ≤20 mm before 24 weeks is vaginal progesterone.
Can a pessary be used instead of progesterone?
SMFM 2024 recommends not placing a cervical pessary for the prevention of preterm birth in patients with a short cervix and a singleton pregnancy. A pessary is not considered a standard alternative to vaginal progesterone.