- Spontaneous preterm birth is associated with elevated cervicovaginal fetal fibronectin levels, especially in women who deliver at an early gestational age.
- A recent study suggests that antibiotic therapy in women with a positive fetal fibronectin screening test does not reduce the incidence of spontaneous preterm birth or improve neonatal outcomes.
- Fetal fibronectin cannot be recommended as a screening test for preterm birth.
This article reviews the landmark studies of fetal fibronectin testing, as well as findings of a recent multicenter double-blind, place-bo-controlled trial of antibiotic therapy.
Approximately two thirds of preterm births are “spontaneous,” associated with preterm premature rupture of the membranes (PPROM) or preterm labor. The remainder have been linked to a variety of other maternal and fetal conditions, such as preeclampsia remote from term, and fetal growth restriction.
Because most studies have focused on identifying women at risk of spontaneous preterm birth, our discussion is limited to this group.
Requirements for routine screening
A screening test is not meant to be diagnostic. Persons with positive findings require more conclusive testing, followed by treatment or another intervention.1
A screening program must meet specific criteria before widespread implementation can be recommended:
- The screened condition poses a significant burden.
- The test is sensitive and specific.
- It is inexpensive and easy to perform.
- It is safe and acceptable to patients.
- Effective treatment is available for patients who test positive.
Cervicovaginal fibronectin is elevated in women who deliver preterm
Fetal fibronectin is a glycoprotein produced by many cell types, including those of the fetal amnion. Indeed, high concentrations of the protein are found in amniotic fluid and maternal plasma.2
Fetal fibronectin acts as an intracellular adhesive; it “glues” the blastocyst to the uterine endometrium. In addition, the glycoprotein is secreted throughout pregnancy, bonding the placenta to the uterus until parturition.
Normally, cervicovaginal secretions contain increased amounts of fetal fibronectin at the beginning of gestation and 1 to 2 weeks before the onset of labor at term. However, in women who deliver preterm, these levels are elevated as early as the second trimester. This observation led early investigators to postulate that fibronectin sampling might be a suitable screening test for increased risk of spontaneous preterm delivery.
Possible causes. It is not clear precisely what causes cervicovaginal fetal fibronectin levels to increase prematurely in women at risk of preterm delivery.
One possibility might be infection: A large percentage of preterm births are associated with subclinical infection and bacterial colonization of the fetal membranes; inflammation associated with such infection may lead to disruption of the extracellular matrix, causing release of fetal fibronectin into the cervix and vagina.
Landmark studies confirm predictive role
In 1991, Lockwood et al2 published one of the first studies to find a predictive association between fetal fibronectin levels and preterm delivery. In 144 women with uncomplicated pregnancies, cervicovaginal concentrations of fetal fibronectin were rarely greater than 50 ng/mL between 21 and 37 weeks of gestation. In contrast, high levels were detected in 95% of 65 patients with PPROM and in 50% of 117 patients with preterm uterine contractions and intact membranes. The 3 groups were well matched for age, race, gravidity, and parity. An elevated fetal fibronectin level identified 60 women who delivered before term with a sensitivity of 81.7% and specificity of 82.5%.
From 1996 to 1998, Goldenberg et al3-5 published several reports on a large multicenter observational study of 2,929 women. In contrast to the Lockwood study, these women were asymptomatic—there was no indication that any would deliver prematurely. Cervicovaginal fetal fibronectin was measured every 2 weeks between weeks 22 to 24 and 30; a test was considered positive if the concentration was 50 ng/mL or greater. The primary outcome was spontaneous delivery associated with PPROM or preterm labor before 35 weeks. The investigators found a significant association between abnormal fetal fibronectin levels and preterm birth (TABLE 1).
Sensitivity was greater for earlier than for later preterm delivery; thus, a positive test would be more useful for identifying women who would deliver before 28 weeks.
Specificity and negative predictive values were much greater than sensitivity and positive predictive values, suggesting that a negative test might be more informative clinically than a positive one. Additionally, sensitivity for earlier preterm delivery (24 to 26 weeks) was greater than for later preterm delivery (28 to 30 weeks); thus, a positive fetal fibronectin test would be more useful for identifying women who would deliver before 28 weeks’ gestation.