The association between cervical excisional procedures, midtrimester cervical length, and preterm birth

OBJECTIVE: To determine whether a prior cervical excisional procedure (a loop electrosurgical excision procedure or cold knife cone) is associated with a short midtrimester cervical length (<3 cm) and whether having a short cervix explains the relationship between this procedure and preterm birth. STUDY DESIGN: In this cohort study of women with a singleton pregnancy who underwent routine cervical length assessment between 18 and 24 weeks of gestation, women with a history of a prior cervical excisional procedure were compared with those without such a history. Bivariable and multivariable analyses were performed to identify whether a prior cervical excisional procedure remained an independent risk factor for preterm birth after controlling for cervical length. RESULTS: Of the 6669 women who met inclusion criteria, 460 (6.9%) had a prior cervical excisional procedure. Mean cervical length was shorter (4.2 +/- 0.9 cm vs 4.5 +/- 0.9 cm, P < .001) and the proportion of women with a short cervix was higher (6.5% vs 1.5%, P < .001) in women with a prior cervical excisional procedure. In multivariable regression, both a short cervix (adjusted odds ratio, 6.19; 95% confidence interval, 3.85-9.95) and a prior cervical excisional procedure (adjusted odds ratio, 1.53; 95% confidence interval, 1.04-2.25) were significantly associated with preterm birth. CONCLUSION: Women with a prior cervical excisional procedure have shorter midtrimester cervical lengths. Both a prior cervical excisional procedure and a short cervix were independently associated with preterm birth. These data suggest that the risk of preterm birth associated with a prior loop electrosurgical excision procedure or cold knife cone is not merely due to postsurgical shortening of the cervix.