Preterm labor is the leading cause of neonatal severe short-term and long-term complications.Although some epidemiological high-risk factors are the main cause，preterm labor occurs in only 10% of women who deliver before 34 weeks of gestation.Whether it is singleton or twin pregnancy，the measurement of cervical length can be performed routinely by transvaginal ultrasonography at 18 to 24 weeks of gestation，and cervical length ≤25mm can be used an a cut-off value to predict preterm labor.