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 greater than or equal to 1 cm (internal os) or effaced greater than 30%, the relative risk of preterm labor was increased to 1.8 to 4.2. Negative predictive values for cervical status were greater than 92%, but positive predictive values were less than or equal to 18%. Change over time was unusual (dilatation increase greater than or equal to 1 cm or effacement increase greater than or equal to 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.