OBJECTIVE: To understand the relationship between cervical length and the risk of prematurity in parous women without a history of preterm delivery.
METHODS: Data from 2,998 singleton pregnancies enrolled in a multicenter, observational cohort study were analyzed. We subgrouped the population into the following categories: those with history of at least one spontaneous preterm birth (n=467); nulliparous (n=1,237); and parous with a history of at least one term birth and no previous preterm birth (low-risk history group, n=1,284). The relationship between cervical length (measured between 22 and 22 6/7 weeks of gestation) and preterm birth was examined using logistic regression. Assuming a 40% risk reduction with the use of vaginal progesterone, we calculated the number needed to screen to prevent one preterm birth.
RESULTS: An inverse relationship between cervical length and risk of preterm birth was demonstrated for each subgroup. A short cervix (15 mm or less) was identified in only 0.93% of the low-risk group participants compared with 3.4% of the previous preterm birth group participants and 2.1% of nulliparous women. The overall rate of preterm birth was lowest (10.5%) in the low-risk history group; however, the rate of preterm birth for these women with a short cervix was 25%. For a cervical length cutoff of 15 mm or less, preventing one spontaneous delivery before 34 weeks of gestation would require screening 167 (95% confidence interval [CI] 112–317) women with a previous preterm birth, 344 (95% CI 249–555) nulliparous women, and 1,075 (95% CI 667–2,500) women at low risk.
CONCLUSIONS: Although ultrasonographic short cervix is a risk factor for preterm birth among parous women with exclusively term births, the incidence of a short cervix is very low. The number needed to screen to prevent one preterm birth is considerably greater for women who have a low-risk obstetric history.
LEVEL OF EVIDENCE: II