Cervical dimensions and risk of preterm birth: A prospective cohort study

Objective: To examine the relation between cervical dilatation and length a nd the risk of spontaneous preterm birth, including its subtypes preterm la bor and preterm premature rupture of membranes (PROM). Methods: Cervical dimensions assessed by clinical examination were recorded prospectively at 24-29 weeks' gestation in 871 subjects with singleton pre gnancies who were followed to delivery. Relative risks (RRs) of preterm bir th, preterm labor, and preterm PROM were calculated for clinically distingu ishable categories of cervical dilatation and length and for cervical score (length minus dilatation). Regression analysis was used to adjust for conf ounding. Time to delivery from baseline examination was summarized using su rvival analysis. Results: There were 73 spontaneous preterm births (8.3%), 46 preterm labors and 27 cases of preterm PROM. All cervical measurements were associated wi th increased risks of preterm birth, with increasing abnormality more stron gly predictive of risk. The adjusted RR for preterm birth with dilatation o f at least 0.5 cm was 2.9 (95% confidence interval [CI] 1.2, 7.3); for leng th of 1.5 cm or less, the RR was 2.1 (95% CI 1.0, 4.5), and for cervical sc ore less than 2.0, the RR was 2.8 (95% CI 1.4, 5.6). The association with c ervical measurements was stronger for preterm PROM than for preterm labor, although precision was limited. These measurements had high specificity (93 -99%) and low sensitivity (8-20%) for predicting preterm birth. Conclusion: In asymptomatic women at 24-29 weeks' gestation, greater cervic al dilatation and shorter length were associated,vith increased risk of spo ntaneous preterm delivery, particularly preterm PROM. (Obstet Gynecol 1999; 93:504-9. (C) 1999 by The American College of obstetricians and Gynecologis ts.).