Twin pregnancies have a high risk on extreme preterm birth (PTB) at less than 28 weeks of gestation, which is associated with increased risk of neonatal morbidity and mortality. An asymptomatic short cervix or dilatation at midpregnancy is associated with high rates of extreme PTB. A possible effective method to reduce these high rates of extreme PTB is a minor operative procedure, the placement of a vaginal cerclage. In the TWIN Cerclage study we assess the effectiveness of a vaginal cerclage in women with a twin pregnancy (below 24 weeks of gestation) and a short cervix (≤ 25 mm) or cervical cilatation compared to the current standard treatment (no cerclage) in the prevention of extreme PTB.