With improving technology, the fetal yolk sac can be routinely visualized sonographically in all living gestations of six to ten weeks. The minimal growth of the yolk sac during this interval and its subsequent obscuration by the growing amniotic sac are verified in this study. An important new sign of missed abortion has been inferred by Bernard and Cooperberg (AJR 144:597, 1985), and is titled in this article the "yolk sac sign." A gestational sac of 25 mm or more in mean diameter and empty except for the yolk sac is highly suspicious for nonviable gestation. This one-year prospective study adds nine such cases. To enhance specificity of this sign, additional criteria specify a yolk sac measuring 4 mm, a free-floating position within the gestational sac, and evacuation of the yolk sac on follow-up scan. However, when a ring-like structure measures 3 mm or less and lies peripherally in the gestational sac, this must be presumed to be a potential fetal pole.