Transvaginal cervical length is only as useful as the technique behind it, and the cut-offs in routine use were established with a specific method. The bladder should be empty, the probe placed in the anterior fornix without pressure that artificially lengthens the canal, and the internal and external os both visible in the same sagittal plane with the whole canal in view.
Take three measurements over a few minutes and report the shortest, because a funnelling cervix can look normal in one frame and short in the next. Transabdominal measurement overestimates length and is not interchangeable with the transvaginal figure, so a reassuring abdominal cervical length in a woman with risk factors should not close the question.
A measurement that is not standardised does not become useful by being repeated. Where you cannot get a technically satisfactory view, say so on the report rather than issuing a number that will be acted on.
- Empty the bladder before measuring — a full bladder lengthens the apparent canal
- Keep the probe pressure light enough that the cervix is not compressed against the posterior wall
- Take three measurements and report the shortest, not the average
- Do not treat a transabdominal cervical length as equivalent to the transvaginal figure
- Record when the view was technically limited, rather than reporting a number you do not trust
Why it matters
A poorly acquired cervical length is worse than none, because it gets acted on with the same confidence as a good one.
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