In the gestational sac, the first structure which is visible is the yolk sac. It is better to see a yolk sac when the gestational sac has grown to measure over 10 mm. The normal size of the yolk sac is round and it measures approximately 6 mm. If the measure grows beyond 6 mm and displays bizarre shape or is calcified, it can be considered abnormal. Pregnancies with such abnormal yolk sac often fail