When is the fetal fibronectin test done?

What does the evidence say about fetal fibronectin test timing during pregnancy?

1. A health-technology assessment describes rapid fetal fibronectin testing from 22 weeks for symptomatic patients, using 50 ng/mL as the usual positive threshold and a 7-to-14-day risk horizon. 1
2. In a large symptomatic cohort summarized in a clinical review, a negative result had a 99.5% negative predictive value for delivery within 7 days, but only 3% of the cohort delivered within that interval and the positive predictive value was 13%. 2
3. Fetal fibronectin and transvaginal cervical length measure different signals; using fetal fibronectin selectively when cervical length is intermediate may improve short-term risk classification, but clinical-utility evidence remains limited. 2
4. Blood, recent intercourse, and cervical manipulation can contribute to false-positive fetal fibronectin results, so collection timing and the full clinical assessment matter. 3
5. In a single-center retrospective cohort of 213 asymptomatic singleton and twin pregnancies with cervical length at or below 25 mm, negative predictive values for delivery within 7 days were 93% to 100%, while positive predictive values were only 7.1% to 25%. 4

Is fetal fibronectin test timing safe in each trimester?

• First trimester. Fetal fibronectin is normally present early in pregnancy, so the test is not used as a first-trimester preterm-labor screen. Prior birth history and later cervical-length assessment guide risk review instead.
• Second trimester. Testing becomes informative after about 22 weeks, most often when contractions, pressure, backache, discharge change, or a selected short-cervix scenario raises concern. The swab should be collected before a digital cervical examination because blood or recent manipulation can create a false-positive result.
• Third trimester. Clinical use is concentrated before about 34 to 35 weeks, when a short-term risk estimate can change transfer, observation, corticosteroid, or other planning. Later in pregnancy, fetal fibronectin naturally reappears as birth approaches and the result becomes less discriminating.