Objective: To compare the effectiveness of cervical pessary and vaginal progesterone in improving long-term developmental outcomes of children born to women with twin pregnancies and mid-trimester short cervical length (CL). Design: Randomized controlled trial (RCT). Setting: My Duc hospital, Ho Chi Minh City, VietNam. Population: Children born to women in an RCT comparing cervical pessary and progesterone (NCT02623881). Methods: At or after 2 years of age, their parents completed a validated Vietnamese version of Ages & Stages Third Edition Questionnaires (ASQ-3) and Red flag questionnaire. Main Outcome Measures: Mean ASQ-3 scores, red flag signs, and the composite outcome of perinatal death or survival with any abnormal ASQ-3 score. Results: In original RCT, we randomized 300 women to pessary (n=150) or progesterone (n=150). After counting the number of perinatal death and lost to follow-up, in pessary group, 116 (82.8%) parents of 229 children returned the questionnaire versus 118 (82.5%) parents of 232 children in progesterone group. Mean ASQ-3 scores of the five skills, as well as red flag signs, did not differ significantly between the two groups. However, the percentage of children having died or having abnormal ASQ-3 scores in fine motor was significantly lower in the progesterone group (1.7% vs 6.1%, P=0.01). In an exploratory analysis, there were no significant differences in the composite outcome of perinatal death or survival with any abnormal ASQ-3. Conclusions: Cervical pessary and progesterone have a comparable effect on developmental outcomes in ≥24 months of children born to women with twin pregnancies and short CL.