Background:The yolk sac is the first anatomical structure identified ultrasonographically within the gestational sac and acts as the primary route of exchange between the human embryo and the mother before the placental circulation is established. Aim of Study:To determine whether yolk sacs with an abnormal sonographic appearance in pregnancies at 5-10 weeks gestation, are associated with adverse pregnancy outcomes or not. Patients and Methods:This study included one hundred pregnant women who are between 5-10 weeks of gestation at Tanta University Hospitals. They were prospectively evaluated concerning for sonographic characteristics of the yolk sacs and perinatal outcomes.Results: An abnormal yolk sac was found in 42 pregnancies. In pregnancies with enlarged yolk sacs, a miscarriage occurred in 71.4% of cases (5/7). The pregnancies with a yolk sac diameter ≥6mm had a significantly higher risk of miscarriage (p=0.001). Miscarriage occurred in 29.4% of pregnancies with irregular yolk sacs (5/17) and 30.8% of pregnancies with echogenic yolk sacs (4/13).Conclusions: An enlarged yolk sac is strongly associated with a significantly increased risk for miscarriage. The presence of an echogenic or irregular yolk sac appears to be unrelated to adverse perinatal outcome.