The Preterm Prediction Study: Sequential cervical length and fetal fibronectin testing for the prediction of spontaneous preterm birth

OBJECTIVES: This study was undertaken to further elucidate the pathogenesis of preterm birth by means of traditional risk factors and new markers for preterm birth derived from the Preterm Prediction Study. STUDY DESIGN: A total of 3076 women (2929 with singleton gestations and 147 with twin pregnancies) were categorized according to the presence of risk factors including black race, low body mass index, the presence of bacteria l vaginosis, and previous preterm birth. At 24 and 28 weeks' gestation cerv ical length was measured and categorized as short (less than or equal to 25 mm) or normal (>25 mm). Vaginal and cervical fetal fibronectin concentrati ons were measured at 24, 26, 28, and 30 weeks' gestation and results were c ategorized as positive (greater than or equal to 50 ng/mL) or negative (<50 ng/mL). RESULTS: At 24 to 26 weeks' gestation women with each of the risk factors w ere more likely to have positive fibronectin test results or to have a shor t cervix. Among women with negative fetal fibronectin results at 24 to 26 w eeks' gestation those with a short cervix were more likely to have positive fetal fibronectin results at 28 to 30 weeks' gestation, and among those wi th normal cervical length those women who had positive fetal fibronectin re sults were more likely to have a short cervix at later evaluation. Most wom en who had positive fetal fibronectin results at 24 to 26 weeks' gestation had negative results at 28 to 30 weeks' gestation, whereas most but not all women who had a short cervix at 24 to 26 weeks' gestation still had a shor t cervix at 28 to 30 weeks' gestation. In each period women with both a pos itive fetal fibronectin result and a short cervix were at substantially inc reased risk of spontaneous preterm birth; women with either marker alone ha d intermediate and approximately equal risks of spontaneous preterm birth, and women without either marker had a low risk of spontaneous preterm birth . CONCLUSION: Regardless of other risk factors, a short cervix predicts a sub sequent positive fetal fibronectin result, and a positive fetal fibronectin result predicts subsequent cervical shortening. These data do not support a single sequence of events leading to spontaneous preterm birth.