Four hundred eighty-six consecutive women who underwent endovaginal sonography when their fetuses were less than 10 weeks menstrual age (MA) were evaluated to establish the normal size and shape of the secondary yolk sac (YS) and to assess the value of YS measurement in predicting pregnancy outcome in the first trimester. A YS diameter more than two standard deviations (SDs) above the mean when compared with the mean gestational sac diameter allowed prediction of an abnormal pregnancy outcome with a sensitivity of 15.6%, a specificity of 97.4%, and a positive predictive value of 60.0%. A YS diameter more than two SDs below the mean allowed prediction of an abnormal outcome with a sensitivity of 15.6%, a specificity of 95.3%, and a positive predictive value of 44.4%. No pregnancy with a normal outcome had a YS diameter of greater than 5.6 mm at less than 10 weeks MA. In six patients, the YS diameter was greater than 5.6 mm. All six had an abnormal outcome. Of seven patients with abnormal YS shape at initial sonography, three had abnormal YS shape at follow-up examinations. All three had an abnormal outcome.