Cervicovaginal fibronectin and cervical length at 23 weeks of gestation: relative risk of early preterm delivery

Objectives To establish the prevalence of cervicovaginal fetal fibronectin positivity at 23 weeks of gestation in a routine population of singleton pr egnancies and determine the relative risk of spontaneous delivery before 33 weeks in women with a fibronectin positive result. Design Prospective clinical study. Setting Inner city antenatal clinic. Population Singleton pregnancies attending for routine antenatal care. Methods Cervicovaginal fetal fibronectin and cervical length were measured at 23 weeks of gestation. The distribution of fibronectin positivity within subgroups according to maternal characteristics was calculated and the rel ative risk of spontaneous delivery before 33 weeks was estimated. Main outcome measures Prevalence of a fibronectin positive result and its r elation to cervical length measurement and spontaneous preterm delivery bef ore 33 weeks. Results Of 5146 women participating in the study, 182 (3.5%) had a fibronec tin positive result and 76 (1.5%) had a cervical length of less than or equ al to 15 mm. Fibronectin positive women were more likely to be Afro-Caribbe an in origin, to have had a previous second trimester miscarriage and to ha ve a short cervix. In the 5068 women who were managed expectantly, the sign ificantly independent relative risk of spontaneous delivery at < 33 weeks w as 46.2 (95% CI 18.8-113.6), for cervical length of less than or equal to 1 5 mm, 8.1 (95% CI 3.8-17.5) for a fibronectin positive result, and 4.4 (95% CI2.2-9.1) for cigarette smoking. Conclusion Fibronectin positivity at 23 weeks of gestation provides useful prediction of pregnancies at risk of spontaneous preterm delivery before 33 weeks, with a relative risk that is twice as high as cigarette smoking, bu t is a sixth of that of cervical length.