Checking the length of the cervical canal is measured by ultrasound. Whether to use a negative or abdominal ultrasound depends mainly on whether the woman is pregnant or not. If the woman is pregnant, it is recommended to hold urine for the abdominal ultrasound. Because vaginal ultrasound may affect the fetus, cause contractions and lead to symptoms of preterm abortion, vaginal ultrasound is not used to measure the length of the cervical canal. If the cervical canal is short, the presence of cervical insufficiency should be considered. If cervical insufficiency is present during pregnancy, spontaneous abortion may occur in the middle of pregnancy, so cervical cerclage should be performed at this time. The cervical cerclage is a vaginal ligation of the cervical canal, which is removed at 36-37 weeks of pregnancy.