Evaluation of additive effect of quantitative fetal fibronectin to cervical length for prediction of spontaneous preterm birth among asymptomatic low-risk women

Introduction: Our objective was to evaluate the possible additive effect of quantitative fetal fibronectin to transvaginal ultrasound cervical length measurement between 18(0/7) and 23(6/7) weeks for prediction of spontaneous preterm birth at <37(0/7) weeks among asymptomatic low-risk women.

Material and methods: A prospective observational study was performed of asymptomatic women with singleton gestations between 18(0/7) and 23(6/7) weeks and no prior spontaneous preterm birth. Women with multiple gestations, rupture of membranes, vaginal bleeding, intercourse or vaginal exam within 48 h of enrollment were excluded. Physicians were blinded to the quantitative fetal fibronectin levels, but the cervical length measurements were made available. The primary outcome was spontaneous preterm birth at <37(0/7) weeks.

Results: Of the 528 asymptomatic low-risk women who were prospectively enrolled, 36 (6.82%) had spontaneous preterm birth at <37(0/7) weeks. Using the receiver-operating characteristic curve, fetal fibronectin value of ≥5 ng/mL was identified as the optimal cut-off for predicting spontaneous preterm birth at <37(0/7) weeks. As compared with cervical length ≥20 mm alone, with the use of cervical length ≤20 mm or quantitative fetal fibronectin ≥5 ng/mL as screening criteria for prediction of spontaneous preterm birth at <37(0/7) weeks; sensitivity improved from 11.11 to 61.11%, specificity decreased from 99.59 to 55.08%, positive predictive value decreased from 66.67 to 9.05%, negative predictive value marginally improved from 93.87 to 95.09% and predictive accuracy decreased from 93.56 to 55.49%.

Conclusions: Although the sensitivity improved, other predictive statistics and predictive accuracy did not improve by the addition of mid-trimester quantitative fetal fibronectin to cervical length measurement. Therefore, addition of mid-trimester quantitative fetal fibronectin to cervical length measurement cannot be recommended at this time for prediction of spontaneous preterm birth at <37(0/7) weeks in asymptomatic low-risk women.