Forty-six studies (31,577 women) were included. Of these, 33 studies were in asymptomatic women and 13 studies were in symptomatic women.
Eight studies met all four criteria for high quality: 6 studies of asymptomatic women with singleton pregnancies and 2 studies of asymptomatic women with singleton pregnancies.
The LRs for positive and negative tests varied according to the cut-offs and gestational age at testing. The area under the summary ROC curve was 0.80 (95% CI: 0.69, 0.91) for singleton pregnancies and 0.67 (95% CI: 0.55, 0.79) for twin pregnancies.
Spontaneous pre-term birth was more accurately predicted by studies testing before 20 weeks and using smaller cervical lengths for both singleton and twin pregnancies.
With the exception of one subgroup (one twin pregnancy subgroup), no heterogeneity was found in the LRs of positive tests for all the subgroups tested. Heterogeneity was detected in some LRs for negative tests.
For asymptomatic women of less than 20 weeks' gestation, when using a cervical threshold of 25 mm and a reference standard of pre-term delivery before 34 weeks (5 studies), the LR for a positive test was 6.29 (95% CI: 3.29, 12.02). This would render a 4.1% pre-test probability into a 15.8% post-test probability. The corresponding LR for a negative test was 0.79 (95% CI: 0.65, 0.95).
Studies showed that the larger the funneling, the more accurately it predicted pre-term birth.
High-quality studies showed similar results to all studies combined.
There were few studies assessing the diagnostic accuracy of cervical sonography.
The LRs for positive and negative tests varied according to the cut-offs and gestational age at testing.
Studies showed that funneling appeared to predict spontaneous pre-term birth.
The funnel plot showed no evidence of publication bias.