Relative risk of preterm birth <35 weeks as a function of cervical length

Data from an asymptomatic general obstetric population evaluated between 22 and 30 weeks of gestation with exclusion of patients with multiple gestations, fetal anomalies, cerclage, or placenta previa. Relative risk is in comparison to patients whose cervical lengths were above the 75^th percentile. In this study, the preterm birth outcome was defined as a birth less than 35 weeks of gestation.