Objective To evaluate whether increasing body mass index (BMI) alters the efficacy of ultrasound-directed cerclage in women with a history of preterm birth. Methods This was a planned secondary analysis of a multicenter trial in which women with a singleton gestation and prior spontaneous preterm birth (17 to 33 + 6 weeks' gestation) were screened for a short cervix by serial transvaginal ultrasound evaluations between 16 and 22 + 6 weeks. Women with a short cervix (cervical length < 25 mm) were randomly assigned to cerclage or not. Linear and logistic regression were used to assess the relationship between BMI and continuous and categorical variables, respectively. Results Overall, in the screened women (n = 986), BMI was not associated with cervical length (P = 0.68), gestational age at delivery (P = 0.12) or birth at < 35 weeks (P = 0.68). For the cerclage group (n = 148), BMI had no significant effect. For the no-cerclage group (n = 153), BMI was associated with a decrease in gestational age at delivery, with an estimated slope of - 0.14 weeks per kg/m2 (P = 0.03; including adjustment for cervical length). This result was driven primarily by several women with BMI > 47 kg/m2. Conclusion In women at high risk for recurrent preterm birth, BMI was not associated with cervical length or gestational age at birth. BMI did not appear to adversely affect ultrasound-indicated cerclage. Copyright © 2012 ISUOG. Published by John Wiley & Sons, Ltd.