Treatment decisions for a short cervix rest on a single number, so how it is measured matters. Use transvaginal ultrasound with an empty bladder, avoid pressing on the cervix with the probe, and measure the closed portion from internal to external os along the canal. Take three measurements over a few minutes and record the shortest good-quality one, watching for dynamic change with gentle fundal pressure or over time. Transabdominal measurement is unreliable for this purpose. A short cervix in the mid-trimester is usually taken as 25 mm or less.
- Use transvaginal, not transabdominal, ultrasound for cervical length.
- Empty the bladder and avoid probe pressure on the cervix.
- Take three measurements and record the shortest good-quality one.
- A mid-trimester cervix of 25 mm or less is the usual threshold for 'short'.
Why it matters
Overestimating length by a few millimetres can withhold progesterone from a woman who would benefit.
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