The yolk sac is the first extra embryonic structure that becomes sonographically visible 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. The yolk sac is a round structure that is made up of an anechoic center bordered by a regular well-defined echogenic rim. It is usually 2-5 mm in diameter. The yolk sac appears at 6 weeks, thereafter increases in size, attains its maximum diameter at 10 weeks and then it starts decreasing in size. It disappears at 12 weeks. Aims and Objectives of the present study is to measure the size of yolk sac in pregnancies of duration 6-12 weeks by Transvaginal sonography and also to measure the inner diameter of yolk sac and correlate it with pregnancy outcome. Materials and Methods: We studied 72 pregnant women of duration 6-12 weeks referred by Department of Obstetrics and Gynaecology. The inner diameter of yolk sac was measured by Transvaginal sonography and its correlation with pregnancy outcome was studied. Observations and Results: The mean yolk sac diameter was noted as 3.7±1.8 mm. The diameter of the smallest yolk sac was 1.25 mm and that of the largest was 8.96 mm. Yolk sac size was normal in 62 (88.57%) cases, it was smaller in size in one (1.4%) case. Further in another 7 (10%) cases, the yolk sac was found to be abnormally enlarged. In these cases where yolk sac was either enlarged or smaller in size, gestation terminated into abortion.