To determine if there is any association between the ultrasound appearance of the histerorrhaphy scar during first trimester transvaginal ultrasound; in patients with a previous csection and the risk of histerorrhaphy dehiscence in the present csection. A retrospective study of cases and controls. Revising 726 patient charts with history of one previous c-section, between July 2004 and January 2009. in all the patients a transvaginal conventional obstetric ultrasound was made between 6 and 10 pregnancy weeks gestation. after the obstetric examination the histerorrhaphy scar was evaluated in a sagital plane measuring the maximum width. All the c-sections were performed between 38–39 weeks by the same operator making the ultrasounds and classifying them into: normal-dehiscence-partial or complete rupture. Patient controls where those with a history of csection with out dehiscence, partial rupture or total rupture at the moment of the csection. The average thickness of the scar was 2.8 mm. All of the patients with a scar smaller than 5mm in weeks 6–10 of pregnancy were classified at the moment of their normal c-sections as normal 99%. there were 2 cases of dehiscence which had an initial measurement of the scar of 8–10 mm respectively. 5 cases of partial rupture had a scar measurement in the first trimester of 15, 15, 18, 20 and 22 mm respectively. The relative indirect risk of uterine rupture is 46% when the width of the uterine scar measured in the first trimester of pregnancy is greater than 5mm. over all the global rate of rupture was 0.96%. There were not cases of complete rupture. We concluded that measuring the scar of the Cesarean section in the first trimester of pregnancy could predict in an adequate manner the risk of histerorrhaphy scar rupture. Values of the uterine scar measured by trans-vaginal inferior to 5 mm will be considered safe and values superior to 15 mm as highly dangerous to allow labor to term in patients with a history of Cesarean section.