Preterm birth (PTB) is the major cause of perinatal morbidity and mortality. Worldwide, about 15 million babies are born too soon every year, causing 1.1 million deaths, as well as short- and long-term disability in countless survivors. Few prognostic tests are available to predict PTB. A short transvaginal ultrasound cervical length (TVU CL) has been shown to be a good predictor of PTB.Different strategies have been adopted for prevention of PTB. The evidence supports the use of vaginal progesterone in singleton pregnancies with short cervix, while cervical cerclage seems to be beneficial only in the subgroup of singleton gestations with both prior spontaneous PTB and TVU CL ≤25mm, and not in singletons without prior PTB, nor in multiple gestations.

However, so far there are no level-1 data on the efficacy of TVU CL screening neither in low risk nor in high risk pregnancy Thus, the investigators aim to assess the efficacy of a policy of TVU CL screening in singleton pregnancy with prior spontaneous PTB

Inclusion Criteria:

• 18-50 years of age
• Singleton gestations
• Women with prior spontaneous preterm birth, defined as spontaneous preterm delivery 16 0/7 - 36 6/7 weeks

Exclusion Criteria:

• multiple gestation
• Ruptured membranes or fetal structural or chromosomal abnormality at the time of randomization
• Ballooning of membranes outside the cervix into the vagina at the time of randomization
• Labor or cerclage in situ at the time of randomization