Interactive Screening Tool
Answer three questions to see personalized guidance based on your cervical length measurement.
Is this a singleton or twin pregnancy?
This summary is generated from the OpenMFM.org cervical length screening tool. Clinical decisions should be made in partnership with your healthcare provider.
What is cervical length?
A plain-language explanation of why this measurement matters.
The cervix is the lower part of the uterus that connects to the vagina. During pregnancy, it stays firm and closed to keep the baby safely inside. As the body prepares for birth, the cervix gradually softens and shortens.
Normally, the cervix measures between 3 and 5 centimeters (30–50 mm) in mid-pregnancy. A cervix shorter than 25 mm before 37 weeks is a sign that the body may be getting ready for birth too early.
Cervical length is measured using transvaginal ultrasound — a small probe placed gently at the entrance to the vagina. It is safe during pregnancy and gives a much more accurate measurement than an abdominal ultrasound.
Screening is typically offered between 18 and 24 weeks of pregnancy.
Interventions explained
Plain-language explanations of the evidence-based options for a short cervix.
What it is: Vaginal progesterone is a hormone medication inserted vaginally each night. It is not a contraceptive. It works locally in the cervix and uterus to help delay labor.
Who it is for: Singleton pregnancies with a cervical length of 20 mm or less, with no prior preterm birth. It may also be used alongside cerclage in higher-risk patients.
How to use it: Insert one 200 mg suppository (or 90 mg bioadhesive gel) vaginally at bedtime. Most patients continue until 36–37 weeks of pregnancy.
What to expect: Minimal side effects — some patients notice mild discharge. It does not cause birth defects. It reduces the risk of early preterm birth but does not eliminate it entirely.
Evidence: The PREGNANT trial (Hassan et al., 2011) showed a 45% reduction in preterm birth before 33 weeks in patients with a CL of 10–20 mm who used vaginal progesterone compared to those who did not.
Important note: Vaginal progesterone has not been shown to benefit low-risk patients with normal cervical length (PROLONG trial, 2019). It is targeted specifically to those with a short cervix.
What it is: Cerclage is a procedure in which a stitch (suture) is placed around the cervix to provide physical support and help keep it closed during pregnancy.
Who it is for: Both conditions must be present: (1) a prior spontaneous preterm birth before 34 weeks, AND (2) a cervical length less than 25 mm in the current singleton pregnancy. Cerclage alone is not recommended for a short cervix without prior preterm birth history.
The procedure: Cerclage is typically performed under spinal or epidural anesthesia in a hospital or outpatient surgical setting. The most common type (McDonald cerclage) uses a suture placed around the cervix through the vagina. It takes about 30 minutes.
Recovery: Most patients go home the same day. Light activity restrictions for 24–48 hours. Pelvic rest (no intercourse) is usually recommended. Heavy lifting is avoided for 1 week. Many patients resume normal daily activities within a few days.
Removal: The cerclage stitch is typically removed at 36–37 weeks, or earlier if labor begins or preterm rupture of membranes occurs. Removal is done in the office and takes only a few minutes.
Evidence: A meta-analysis by Berghella et al. found that cerclage combined with vaginal progesterone reduces recurrent preterm birth by approximately 30% in patients with prior sPTB and a short cervix.
Types: The McDonald technique is the most common. The Shirodkar technique is a slightly deeper placement sometimes used in specific cases. Transabdominal cerclage is reserved for cases where transvaginal cerclage is not feasible.
Serial cervical length checks: After a short cervix is identified, repeat transvaginal ultrasound measurements are done every 2–4 weeks to track any further shortening.
Antenatal corticosteroids (ACS): If delivery before 34 weeks appears likely, a short course of corticosteroid injections (betamethasone) is given to help the baby's lungs mature. This is one of the most effective interventions available for improving newborn outcomes after preterm birth. The timing of this discussion depends on gestational age and rate of cervical change.
Magnesium sulfate for neuroprotection: If delivery before 32 weeks appears imminent, magnesium sulfate is administered intravenously to reduce the risk of cerebral palsy in the newborn. This is given in the hospital.
Hospital and NICU counseling: If the pregnancy is remote from term (especially before 28 weeks) and delivery appears possible, your provider may arrange a tour of the neonatal intensive care unit (NICU) and a meeting with a neonatologist so you can understand what to expect.
Activity restriction: Bed rest and activity restriction have not been shown in clinical trials to prevent preterm birth and are not routinely recommended. Discuss any specific activity limitations with your provider based on your individual situation.
Warning signs — call your provider
Know when to reach out right away, especially if you have a short cervix.
- Pelvic pressure or heaviness before 37 weeks — a feeling that the baby is "pushing down" or "falling out"
- Regular contractions — tightening or cramping that comes and goes more than 4 times in an hour
- Low back pain that comes and goes in a rhythmic pattern, especially if new or different from usual
- Change in vaginal discharge — more than usual, watery, mucus-like, bloody, or a sudden gush of fluid
- Abdominal cramping with or without diarrhea before 37 weeks
- Decreased fetal movement — fewer than 10 movements in 2 hours after 28 weeks (kick count)
A note on the cervical pessary
An intervention used in other countries, but not currently recommended in the US.
The cervical pessary (Arabin pessary) is a ring-shaped silicone device placed around the cervix. It is widely used in several European countries for short cervix management.
However, the PROLONG trial (2019), a large randomized controlled trial conducted in low-risk singleton pregnancies, found no benefit from the pessary compared to placebo. Based on this and related evidence, SMFM and ACOG do not recommend the cervical pessary for routine use in the United States for singleton pregnancies with a short cervix.
Research is ongoing, including trials examining pessary use in twin pregnancies and other specific populations. Discuss with your MFM specialist if you have questions about emerging evidence in this area.
Guideline references
All recommendations are grounded in peer-reviewed guidelines and clinical trials.