Results
We identified 108 twin pregnancies screening with cervical dilation 1-6cm at 18–26 weeks of gestation who met inclusion criteria. Some twins were identified when cervical length was suspected shorten by transvaginal ultrasound, which prompted a physical exam. At last, 48 women underwent a combination of McDonald-Shirodkar technique(cases) and 60 were followed up McDonald techniques(controls). To minimize confounding factors, we elucidated the causality structure using a DAG (Figure 1) and performed 1:1 case-control Matching. After re-matching, 42 cases and 42 controls were obtained. Participant demographic characteristics were similar for each group (Table 1).