Both quantitative vaginal fetal fibronectin and serial transvaginal ultrasound cervical length, 2 methods in predicting preterm delivery in first-time pregnancies, had low predictive accuracy for spontaneous preterm birth among nulliparous women, a recent study found. The cohort study included >9,000 US women between October 2010 and May 2014. Transvaginal cervical length and quantitative vaginal fetal fibronectin levels were reviewed at 2 study visits 4 or more weeks apart. Spontaneous preterm birth at <37 weeks was the primary outcome. Researchers found:

• Of 9,410 women (median age 27 years; 60.7% non-Hispanic white), 8,601 (91.4%) had term births, while only 474 (5.0%) had spontaneous preterm births, and 335 (3.6%) had medically indicated preterm births.
• Among women with spontaneous preterm birth, cervical length of ≤25 mm occurred in 35 of 439 (8.0%) at 16 to 22 weeks gestation and in 94 of 403 (23.3%) at 22 to 30 weeks gestation.
• Fetal fibronectin levels of ≥50 ng/mL at 16 to 22 weeks identified 30 of 410 women (7.3%) with spontaneous preterm birth and 31 of 384 (8.1%) at 22 to 30 weeks.