We measured the cervical length of 160 normal pregnant women between 8 and 36 weeks' gestation by vaginal ultrasonography. Normal pregnant women were assigned to one of five groups according to gestational age at the time of the measurements. Cervical length was 34.7±1.3(group 1=8 to 13 weeks), 36.5±2.5(group 2=14 to 20 weeks), 41.7±2.3(group 3=21 to 25 weeks), 38.4±1.8(group 4=26 to 31 weeks) and 32.7±2.0 mm(group 5=32 to 36 weeks). Cervical length of group 1 to 5 was significantly longer than that of control group(nonpregnant women). We found that cervical length increases progressively up to 21 to 25 weeks' gestation and then decreases significantly at 32 through 36 weeks' gestation. Cervical length was normalized within postpartum 1 month. Normogram of cervical length throughout pregnancy was established. This may aid early diagnosis and management of preterm labor and cervical incompetance.