Premature effacement of the cervix, or short cervix, in the midtrimester of pregnancy currently represents the strongest predictor of spontaneous preterm birth. Numerous studies have examined the relationship between short cervix and preterm birth, and although differences in sensitivity and predictive value exist, the overwhelming consensus indicates that the risk of preterm birth increases with decreasing cervical length. Cutoff values for cervical length range from less than 25 mm to less than 15 mm depending on patient characteristics and gestational age. Importantly, short cervix is more sensitive for predicting earlier forms of prematurity (at <32 weeks) than later forms of prematurity (>32 weeks). Cervical length measured at 18 to 24 weeks' gestation appears to be the most valuable measurement. Cervical length assessments can be performed by digital examination, a cervical measurement device, transabdominal ultrasound, or transvaginal ultrasound. Transvaginal ultrasound is the most reliable method to evaluate cervical length, but it is not available in all locations. Current clinical practice standards do not recommend universal screening for cervical length; however, results of randomized controlled clinical trials provide evidence to support expanded screening programs by individual practitioners.