The cervix as a predictor of preterm delivery in 'at-risk' women.

OBJECTIVE: To examine the relationship between ultrasound-determined cervical status and pregnancy outcome in women 'at-risk' of spontaneous preterm delivery.

DESIGN: A prospective cohort study of 120 pregnant women considered to be 'at-risk' of spontaneous preterm delivery by their clinician. Transvaginal ultrasound of the cervix was used to assess overall cervical length, closed endocervical canal length, diameter and internal os dilatation in the second trimester. The main outcome measure was occurrence of spontaneous preterm birth (< 34 and < 37 weeks of gestation).

RESULTS: The overall preterm delivery rate (< 37 weeks gestation) in these women was 35% (n = 42) with 20% (n = 24) delivering < 34 weeks gestation. Of the 71 women with a normal cervix, 8 (11%) delivered < 34 weeks, whereas of the 49 women with an abnormal cervix, 16 (33%) delivered < 34 weeks (RR 2.90; 95% CI 1.35-6.24). Using linear regression, closed endocervical canal length of < 21 mm before 20 weeks is associated with delivery < 34 weeks in 95% of women, and with delivery < 37 weeks in 95% of women if the canal length is < 33 mm. Logistic regression showed closed endocervical canal length to be the only significant factor in those women who delivered < 34 weeks after controlling for possible confounders.

CONCLUSIONS: A strong relationship is demonstrated between cervical status and pregnancy outcome, particularly the cervical findings before 20 and 24 weeks of gestation. The length of the closed portion of the endocervical canal is the best predictor. A beneficial effect of this approach to 'at-risk' women is the reduction in unnecessary interventions in those with normal cervical findings.