Abstract: Objective To explore the relationship between diameter and shape of yolk sac(YC)and early spontaneous abortion. Methods Totally 517 cases of singleton pregnant women were detected by abdominal ultrasound and were followed up.All cases were divided into the normal pregnancy group,threatened abortion group and spontaneous abortion group according to the pregnancy outcomes.The size and shape of yolk sac,the inner diameter of gestational sac were recorded. Results Ultrasound could detected the yolk sac at 5 weeks of pregnancy.The inner diameter of gestational sac was the largest at 9 week and then the diameter was narrowed and disappeared at 12 week.The changes of inner diameter of the gestational sac showed a square relation to gestational time with a regression equation of Y=2.290+0.09X0.001X² and a coefficient of R²=0.634(P=0.000).There were 10 cases with no yolk sac detected,32 cases with the size less than normal,11 cases with normal size,9 cases with the size larger than normal,7 cases with deformation,7 cases with calcification,and 3 cases with abnormal echo.The anomalies of shape and size of yolk sac for threatened abortion and spontaneous abortion group were different and the difference was statistically significant(P<0.05).logistic regression analysis showed that the detection of the yolk sac,the size and shape of yolk sac were independent risk factors for abortion. Conclusion The size and shape of yolk sac were closely related to the pregnancy outcomes.The yolk sac detection by ultrasound at the early stage of pregnancy was a predictor of pregnancy outcome.