Abstract： Objective To study the value of cervical length(CL) by transvaginal sonography in the mid-trimester and late-trimester for the prediction of preterm delivery.Methods The CL was measured by transvaginal sonography for 5277 pregnant women between 22-24 weeks and 28-32 weeks gestation,who were prenatal cared and delivered at the First Hospital of Peking University from June 2008 to November 2009.The pregnancy outcomes were followed,and the relationship between CL and preterm delivery and preterm premature rupture of membrane was studied.Results ( 1 ) The incidence of preterm delivery was 5.4％ (289/5370) total,among of them the incidence of therapeutic preterm delivery was 1.7％ (93/5370),spontaneously preterm delivery was 1.2％ (62/5370),and preterm premature rupture of membrane was 2.5 ％ (134/5370).There are 4 cases (4/5370) who occured late abortion.(2) Excluding the 93 women who had therapeutic preterm delivery,the mean CL of 22-24 weeks was ( 38.8±4.0) mm.The relative risk for preterm delivery when the CL＜30 mm was 5.2,when CL＜25 mm,the relative risk was 11.1,and when CL ＜15 mm the relative risk for preterm delivery was 13.8.The average CL during 28-32 weeks of gestation was ( 34.6±4.8) mm,was significantly shorter than that of 22-24 weeks ( P＜0.05 ).During this period the relative risk for preterm delivery when the CL＜30 mm was 6.9,when CL＜25 mm,the relative risk was 11.1,and when CL＜15 mm the relative risk for preterm delivery was 20.0.(3) A CL＜30 mm as the cutoff value for predicting preterm delivery during 22-24 weeks of gestation has only a 3％ sensitivity and 19％ positive predictive value,but had a 99％ specificity and 96％ negative predictive value.The sensitivity,positive predictive value,specificity and negative predictive value for a CL ＜ 30 mm as the cut-off value for predicting preterm delivery during 28-32 weeks of gestation was 33％,21％,95 ％ and 97 ％ respectively.(4) The total number of preterm premature rupture of membrane pregnant women was 134 (2.5％ ),who had a mean CL of (38.4 ±4.7) mm during 22-24 weeks of gestation,was similar with the women without preterm premature rupture of membrane ( PPROM),but during 28-32 weeks of gestation the women who occured PPROM had a mean cervical length of ( 30.6 ± 8.1 ) mm,and was significantly shorter than that of women without PPROM ( 34.7 + 4.6 ) mm.Conclusions ( 1 ) CL in 28-32 weeks of gestation issignificantly shorter than that of in the mid-gestation,but more than 90％ of women has a CL≥30 mm.(2)The shorter the CL is,the greater the relative risk of preterm delivery.According to different CL for clinical consulting objective relative risk could be provide.(3) The CL during 28-32 weeks of gestation can also predict preterm delivery,the sensitivity is obviously better than that of 22-24 weeks of gestation.(4) The CL during 28-32 weeks of gestation is valuable for predicting of PPROM.