This will be a multicenter trial in the UK and other countries. During routine ultrasound scan at 20-24 weeks of gestation for examination of fetal anatomy and growth, all women with twin pregnancy or with singleton pregnancy found to have a cervix of <25 mm in length and where the fetuses are found to be alive with no major abnormalities, severe twin to twin transfusion syndrome or severe fetal growth restriction in one of the fetuses (in the case of twin pregnancy), will be invited to participate in a randomized trial of standard management vs vaginal insertion of a cerclage pessary. For singleton pregnancy, in both arms the patients with cervical length <15 mm will be given prophylactic progesterone (200 mg vaginal capsule per night up to 34 weeks). Randomization and insertion of the pessary (in those allocated to this group) will be carried out within 5 days after the 20-24 weeks scan.