Cervical Stiffness Assessment in High-risk Pregnancies for Preterm Birth

Inclusion Criteria: * Informed consent signed by the participant * Pregnant women more or equal to 18 years of age. * Singleton gestation * Weeks of pregnancy between 16 weeks 0 days and 32 weeks and 0 days * Cervix less or equal to 25 mm (inferior to the 5th centile at 25 wp) * No regular contractions * Planned progesterone treatment Exclusion Criteria: * Cognitively impaired adults * Cerclage or pessary on place * Ongoing progesterone (if continued or started after 13+0 wp) treatment * Multiple pregnancies * Placenta praevia totalis with hemorrhage (irrespective of severity) * Vasa praevia * Light bleeding (if the bleeding can be stopped it is no longer an exclusion criterion) * Heavy vaginal bleeding, hematoma or chorioamniotic membrane separation * Rupture of membranes (exclusion with ROM+ test if PPROM is suspected) * History of cervical surgery only if visible scaring at 12 o'clock * Cervical dilation more equal to 3 cm * Signs of chorioamnionitis * Symptomatic genital infection * Known carrier of HIV or Hepatitis B/C * Fetal malformations