Cervical Length and Fetal Fibronectin
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Direct answer
Cervical length measured transvaginally and fetal fibronectin testing are predictors used to estimate the risk of spontaneous preterm delivery. A cervical length below about 25 mm on mid-trimester scanning defines a short cervix, and a positive fetal fibronectin test in symptomatic women suggests delivery within about two weeks. A negative fetal fibronectin result is more useful than a positive one, because it has a high negative predictive value.
What you must remember
- Cervical length: measured by transvaginal ultrasound, the reference method, in the mid-trimester at about 18 to 24 weeks.
- Short cervix: commonly defined as below 25 mm, with some guidelines using thresholds nearer 20 mm.
- Funnelling: a funnel-shaped internal os is a marker of higher risk.
- Fetal fibronectin: a glycoprotein from the chorion-decidua junction, sampled from the posterior vaginal fornix on a swab.
- Positive result: in symptomatic women between 24 and 34 weeks, suggests delivery within about 7 to 14 days.
- False positives: vaginal bleeding, semen, digital examination and intercourse within 24 hours.
A pathway from scan to swab
These tests are used in sequence, not in parallel. Each step limits when the next test is valid, so examiners expect the order to be right.
- Measure the cervix: perform a transvaginal scan and record the length of the closed cervical canal in millimetres.
- Assess funnelling: note whether the internal os is funnelled, since funnelling adds to the risk estimate.
- Check eligibility for fibronectin: test only women with symptoms of preterm labour between 24 and 34 weeks, with intact membranes, no bleeding and a cervix not dilated beyond about 3 cm.
- Collect correctly: take the sample from the posterior fornix, avoiding digital examination and intercourse within 24 hours.
- Act on combined findings: a negative fibronectin result reassures against imminent delivery, while a positive result prompts steroid and admission decisions.
Cervical length guides who needs preventive progesterone or cerclage, while fibronectin guides short-term admission decisions in symptomatic women.
Where the exam tries to catch you
Questions here test the population in which each test is valid and the causes of false results.
- Trap: Using fetal fibronectin as a routine screen in asymptomatic women. Correct: The test is used in symptomatic women with signs of preterm labour, where its predictive value is meaningful.
- Trap: Trusting a positive fibronectin result without checking for bleeding or recent intercourse. Correct: These cause false positives, so the test is interpreted with the clinical context.
- Trap: Using cerclage for an isolated short cervix in a singleton with no history. Correct: Progesterone is the usual preventive option for an isolated short cervix on scan.
Frequently asked questions
What cervical length is considered short?
A cervical length below about 25 mm on transvaginal scanning in the mid-trimester is usually called short. Guidelines differ, and some use thresholds near 20 mm. Cervical length measured at 18 to 24 weeks is the basis for progesterone decisions.
Why is fetal fibronectin not used for routine screening?
Fetal fibronectin testing has useful predictive value only in women with symptoms of preterm labour. Its positive predictive value is poor in asymptomatic women. Routine fibronectin screening is therefore not recommended.
What falsely raises a fetal fibronectin result?
Vaginal bleeding, recent intercourse, digital cervical examination and semen can produce a false positive fetal fibronectin result. Sampling is avoided within 24 hours of intercourse. The result is always interpreted alongside the clinical picture.
What does a negative fetal fibronectin test mean?
A negative fetal fibronectin test makes delivery within the next one to two weeks unlikely. Its high negative predictive value is the main clinical use. A negative result does not exclude later preterm delivery, so surveillance continues.