Fetal fibronectin: A protein produced during pregnancy and the basis of a test for preterm delivery. Fetal fibronectin (fFN) functions as a "glue" attaching the fetal sac to the uterine lining. The presence of fFN during weeks 22-34 of a high-risk pregnancy, along with symptoms of labor, suggests that the "glue" is disintegrating ahead of schedule and raises the possibility of a preterm delivery. To test fFN, a cotton swab is used (as in a Pap smear) to collect samples of cervico-vaginal secretions. A negative fFN test result is a highly reliable predictor that delivery will not occur within the next 2 weeks. The American College of Obstetricians and Gynecologists (ACOG) does not recommend the test for routine screening, as its use has not been shown to be clinically effective in predicting preterm labor in low-risk, asymptomatic pregnancies. ACOG recommends fFN testing only for symptomatic, high-risk pregnancies, where preterm labor is suspected.