OBJECTIVE: To determine changes in length of incompetent cervices after cerclage, using transvaginal ultrasound. METHODS: Patients were enrolled in a prospective, observational study under an Institutional Review Board-approved protocol. McDonald or Shirodkar sutures were placed according to physician preference. Pre- and postcerclage cervical lengths were measured within 72 hours of the procedure. At each examination, the first measurement was discarded, and a mean of the subsequent three measurements was calculated. RESULTS: Twenty-one Shirodkar and ten McDonald operations were done. The mean (+/- standard deviation) precerclage cervical length was 2.7+/-0.9 cm and the postcerclage cervical length was 3.6+/-0.9 cm (P