Management of Threatened Preterm Labor by Cervical Length-based Approach or Conventional Approach

Inclusion Criteria: * Pregnant with symptoms of threatened preterm labor which refer to regular uterine contractions at least 1 time in every 10 minutes and persist for 30 minute without cervical dilatation or cervical dilation ≤ 1 cm and \< 80% of effacement. * Age ≥ 18 years * Singleton pregnancy * 24 0/7 - 33 6/7 weeks of gestation * Uneventful antenatal care or low-risk pregnancies Exclusion Criteria: * The definitive diagnosis of true labor * Proceed to the active phase of labor * Age \< 18 years * Multiple pregnancies * \< 24 or ≥ 34 weeks of gestation * Fetal chromosomal or structural abnormalities * Abnormal fetal growth * Abnormal amniotic fluid * Evidence of placenta previa * Significant vaginal bleeding that cannot be ruled out placenta previa * Evidence of rupture of membranes * Evidence of intrauterine infection * Evidence of uterine malformation * Evidence of lethal fetal malformation * Unable to communicate or understand the information of the study