LOW CONCENTRATIONS OF VAGINAL FETAL FIBRONECTIN AS A PREDICTOR OF DELIVERIES OCCURRING AFTER 41 WEEKS

OBJECTIVE: This study sought to determine whether low concentrations o f fetal fibronectin in the cervical and vaginal secretions of patients at 39 weeks' gestation predicted pregnancies progressing beyond 41 we eks' gestation. STUDY DESIGN: A retrospective cohort study was underta ken using cervical and vaginal samples collected from 75 consenting pa tients during the thirty-ninth week of gestation. Levels of fetal fibr onectin were measured by immunoassay. Demographic, obstetric, neonatal , and laboratory data were analyzed by Fisher's exact test, Student t test or Wilcoxon's rank-sum test, multiple logistic regression, and re ceiver-operating characteristic curve analysis. RESULTS: There was a 3 5.5-fold increase in vaginal fetal fibronectin concentrations among pa tients delivered at < 41 weeks compared with those delivered at greate r than or equal to 41 weeks. The receiver-operating characteristic cur ve analysis indicated that the optimal fetal fibronectin predictor of prolonged pregnancies was a vaginal fetal fibronectin value < 60 ng/ml present between 39 weeks 0 days and 39 weeks 6 days' gestation (sensi tivity 95.7%, 95% confidence interval 87.3% to 100.0%; specificity 44. 2%, 95% confidence interval 30.7% to 57.7%; positive and negative pred ictive values 43.1% [95% confidence interval 29.5% to 56.7%] and 95.8% [95% confidence interval 87.8% to 100.0%], respectively). The relativ e risk for a prolonged pregnancy resulting from a vaginal fetal fibron ectin value < 60 ng/ml was 10.4 (1.5 to 72.4). Among patients with a v aginal fetal fibronectin value greater than or equal to 60 ng/ml, 80.8 % were delivered within 1 week and 92.3% within 10 days of sampling. I n contrast, among patients with vaginal fetal fibronectin value < 60 n g/ml 63.3% remained undelivered after 1 week. After parity and cervica l dilation were controlled for, multiple logistic regression demonstra ted that a vaginal fetal fibronectin value < 60 ng/ml was a significan t independent predictor of pregnancies delivered at greater than or eq ual to 41 weeks (adjusted odds ratio 12.8 [95% confidence interval 1.5 to 107.2]). CONCLUSIONS: A vaginal fetal fibronectin value greater th an or equal to 60 ng/ml at 39 weeks' gestation is predictive of delive ry within 10 days, whereas values < 60 ng/ml identify 95% of pregnanci es progressing past 41 weeks' gestation. This data may allow for the p rediction of postdates pregnancies, thus facilitating appropriate obst etric interventions.