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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Preterm Labor and Preterm Premature Rupture of Membranes
Sponsored by The Indian Health Service Clinical Support Center
7. Ultrasound: Cervical length
Another possibly helpful ancillary investigation, if available at your institution, is a transvaginal ultrasonographic cervical length . It is not an easy sonographic exam however, so if you don't have access to an experienced sonographer at your facility, this section may not be pertinent, or you may want to have someone trained.
Transvaginal sonography for cervical length is not recommended for screening asymptomatic women because of its low positive predictive value in this population. However, it may be a helpful adjunct for triage of symptomatic women. The median (50 th percentile) cervical length is 3.5 + 0.8 cm in the third trimester. The “short cervix”, less than the 10 th percentile, measures less than 2.5 cm in the late trimester.
Transvaginal sonography also gives you an insight into the condition of the internal cervical os which you often cannot palpate. Cervical “funneling” refers to a shortening of the overall cervical length by prolapse of the amniotic membranes into the upper cervical canal in a “V” or “U” configuration. Significant “funnels” are 1.5 x 1.5 cm in their widest dimensions, and their presence increases your level of suspicion for true PTL rather than just “POOC”.
Cervical lengths greater than 3.0 cm have a negative predictive value for preterm delivery of 89-93 per cent, while cervices shorter than 2.0 cm have a positive predictive value for delivery within the next seven days of about 15-18 per cent, values similar to those obtained with fFN determinations. Combining fFN and cervical length enhances the sensitivity of your predictions. So a woman with minimal cervical change on exam, a negative fFN , and a cervix >2.5 cm may be a candidate to go home. On the other hand, the same woman with unimpressive cervical change, but who has a positive fFN and a cervix shorter than 2.5 cm, would be a good candidate for admission or transport, other clinical variables being equal.
