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Women at high risk for spontaneous preterm birth and second-trimester loss who have had:
- a previous spontaneous preterm birth <36 weeks
- a previous spontaneous second-trimester loss 16–24 weeks
- a previous large loop excision of the transformation zone (LLETZ) procedure with known depth of excision ≥10 mm
- a knife cone biopsy or trachelectomy or more than one LLETZ procedure
- a known uterine or cervical anomaly, such as unicornuate uterus.
Routine cervical length scanning at the time of the mid trimester anatomy scan is not currently recommended. For more detail, see the NZMFMN statement Routine measurement of cervical length at time of mid trimester anomaly scan in all women (PDF, 287 KB) (NZMFMN 2011b).
In the event of finding a short cervix before 24 weeks, specialist consultation should be sought.
Required clinical details
- Dating information
- Risk factors for spontaneous preterm birth/second-trimester loss (see Indications above)
- Any symptoms.
For high-risk women, cervical screening should be performed every two weeks, from 16 up to 24 weeks.
Cervical imaging before 16 weeks or after 24 weeks may be required rarely, in some very high-risk individuals (at specialist discretion).
These scans can be challenging to interpret and are best performed in specialised units.
A TV scan is required for accurate cervical length measurement (TA assessment with a full bladder falsely elongates the cervical length).
- Perform TV scan with an empty bladder.
- Angle the ultrasound probe gently towards the anterior fornix. Avoid exerting pressure on the cervix as this may falsely increase the apparent length of the cervix.
- Obtain a sagittal view of the cervix, using the endocervical mucosa (which may be of increased or reduced echogenicity compared with the cervix) as a guide to the true position of the internal os, as opposed to the lower segment of the uterus.
- Measure the linear distance between the triangular area of echodensity at the external os and the V-shaped notch at the internal os.
- Scan over a period of 2–3 minutes as the cervical length may change over time due to uterine contractions.
- Record the shortest measurement.
- Apply suprapubic and fundal pressure and record the shortest closed cervical length measurement.
- Funnelling is observed sonographically as dilatation of the internal os. The length of funnelling may be measured as the length of the open cervix from the level of the internal os to the medial point of the closed cervix. (Note: Documentation of width of funnelling is not clinically helpful.) Most women with a short cervix will have funnelling of the internal os. Reporting the presence of funnelling may aid clinicians in planning care, but the shortest closed length of the cervix is the only validated measure for predicting risk of preterm birth.
- If the cervix is obviously open with bulging membranes, perform TA and translabial scan if possible. TV scan should not be performed without first discussing it with the referrer.
- Third-trimester cervical length assessment is not recommended.
For further information, please see the FMF webpage: Education: Cervical assessment.
Reporting guide and referral recommendations
Minimum reporting requirements
- Dating information, by earliest scan
- Fetal biometry, if appropriate
- Cervical length in mm, and any change with application of suprapubic/fundal pressure (Note: A measurement of <25 mm at <24 weeks gestation is considered to be a short cervix.)
- Length of funnelling
- If the cervix is completely open with no measurable cervical length, document these findings, along with presence of bulging of fetal membranes into the vagina or retained in the cervical canal
- Recommend follow-up in 2 weeks (high-risk women only) if the cervix is normal in length
- Urgent same-day specialist review is recommended for an open cervix.
- Open cervix ± bulging membranes
- Short cervix <25 mm.