The preterm cervix and preterm labor: Relative risks, predictive values, and change over time

The accurate prediction and diagnosis of preterm labor continue to frustrate the clinician. This is partly due to a scarcity of cervical data from the early third trimester. A total of 760 prospective, serial, paired, and blinded pelvic examinations were done at 28 to 34 weeks of gestation for 191 patients without a history of preterm labor. If the cervix was dilated ⩾1 cm (internal os) or effaced >30%, the relative risk of preterm labor was increased to 1.8 to 4.2. Negative predictive values for cervical status were >92%, but positive predictive values were ⩽18%. Change over time was unusual (dilatation increase ⩾1 cm or effacement increase ⩾40%), suggesting that a baseline late second-trimester examination could assist in the early but accurate diagnosis of preterm labor should it be suspected later in gestation. These data suggest that even in the low-risk patient, an early cervical examination could be beneficial.