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Objectives To establish the prevalence of cervicovaginal fetal fibronectin positivity at 23 weeks of gestation in a routine population of singleton pregnancies 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 relative risk of spontaneous delivery before 33 weeks was estimated.

Main outcome measures Prevalence of a fibronectin positive result and its relation to cervical length measurement and spontaneous preterm delivery before 33 weeks.

Results Of 5146 women participating in the study, 182 (3.5%) had a fibronectin positive result and 76 (1.5%) had a cervical length of ≤ 15 mm. Fibronectin positive women were more likely to be Afro-Caribbean in origin, to have had a previous second trimester miscarriage and to have a short cervix. In the 5068 women who were managed expectantly, the significantly independent relative risk of spontaneous delivery at < 33 weeks was 46.2 (95% CI 18.8–113.6), for cervical length of ≤ 15 mm, 8.1 (95% CI 3.8–17.5) for a fibronectin positive result, and 4.4 (95% CI 2.2–9.1) 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, but is a sixth of that of cervical length.