The length of the cervix becomes shorter after pregnancy from the 7th to 8th month. After pregnancy the cervix becomes congested and edematous with hormones, and the glands in the cervical canal become hyperplastic and hypertrophied, causing the cervix to gradually soften from the early stages of pregnancy. The cervix is able to maintain the stability of pregnancy during gestation, so that the closure of the cervix during pregnancy is maintained until full term. Normally the length of the cervix is in stable change from 14 to 28 weeks of gestation, conforming to a bell-shaped curve. After 28 to 32 weeks of gestation, the cervix length gradually shortens, and after labor the cervical canal disappears and the uterine opening dilates, allowing the fetal head to pass through. When the cervical length of pregnant women at 16~24 weeks of gestation ≦25mm, there may be a risk of preterm labor, which should be taken seriously. When the cervical canal is too short, cervical cerclage can be used to prevent preterm labor. In the clinic, it is recommended that pregnant women with short cervical canal should pay attention to bed rest during pregnancy, avoid doing heavy physical labor, avoid walking, stand as little as possible, avoid fatigue and so on.