The objective of our study was to investigate the relationship between sonographic findings and the occurrence of abortion in pregnancies complicated by first-trimester bleeding in which fetal cardiac activity was documented upon admission. A prospective study of transvaginal sonography was performed in 270 pregnant patients with bleeding between 5 and 12 weeks' gestation. The study group included 149 cases in which a singleton fetus with cardiac activity was initially documented. The outcome variable was pregnancy loss prior to 20 weeks. The influence of sonographic findings on admission was studied by univariate analysis and logistic regression. The prevalence of abortion was 23/149 (15%). A significant relationship (p < 0.05) was found between the occurrence of abortion and the following: fetal bradycardia (heart rate less than -1.2 SD from the mean), a discrepancy between the diameter of the gestational sac and crown-rump length less than -0.5 SD from the mean, and a discrepancy between menstrual and sonographic age of more than 1 week. According to the logistic regression equation that was obtained, the probability of abortion in first-trimester bleeding with documented fetal cardiac activity upon admission varied between a minimum of 6% when none of the above risk factors were present and a maximum of 84% when all were present. The presence of any of the above factors identified 84% of all subsequent abortions.