Low concentrations of vaginal fetal fibronectin as a predictor of deliveries occurring after 41 weeks

Abstract

This study sought to determine whether low concentrations of fetal fibronectin in the cervical and vaginal secretions of patients at 39 weeks' gestation predicted pregnancies progressing beyond 41 weeks' gestation. A retrospective cohort study was undertaken using cervical and vaginal samples collected from 75 consenting patients during the thirty-ninth week of gestation. Levels of fetal fibronectin 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 receiver-operating characteristic curve analysis. There was a 35.5-fold increase in vaginal fetal fibronectin concentrations among patients delivered at < 41 weeks compared with those delivered at > or = 41 weeks. The receiver-operating characteristic curve 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 (sensitivity 95.7%, 95% confidence interval 87.3% to 100.0%; specificity 44.2%, 95% confidence interval 30.7% to 57.7%; positive and negative predictive values 43.1% [95% confidence interval 29.5% to 56.7%] and 95.8% [95% confidence interval 87.8% to 100.0%], respectively). The relative risk for a prolonged pregnancy resulting from a vaginal fetal fibronectin value < 60 ng/ml was 10.4 (1.5 to 72.4). Among patients with a vaginal fetal fibronectin value > or = 60 ng/ml, 80.8% were delivered within 1 week and 92.3% within 10 days of sampling. In contrast, among patients with vaginal fetal fibronectin value < 60 ng/ml 63.3% remained undelivered after 1 week. After parity and cervical dilation were controlled for, multiple logistic regression demonstrated that a vaginal fetal fibronectin value < 60 ng/ml was a significant independent predictor of pregnancies delivered at > or = 41 weeks (adjusted odds ratio 12.8 [95% confidence interval 1.5 to 107.2]). A vaginal fetal fibronectin value > or = 60 ng/ml at 39 weeks' gestation is predictive of delivery within 10 days, whereas values < 60 ng/ml identify 95% of pregnancies progressing past 41 weeks' gestation. This data may allow for the prediction of postdates pregnancies, thus facilitating appropriate obstetric interventions.