The Human yolk sac has long been considered a vestigial organ, an evolutionary remnant. In the last decade, however, it has been discovered that the human yolk sac plays an active and crucial role during organogenesis. Due to an absent maternal intervillous circulation during the first 12 weeks of pregnancy, the concept of transport of nutrients and oxygen to the embryo must be thoroughly reconsidered. Here the yolk sac plays an essential role by its active and passive transport to the embryo, and by its production of necessary substances. Animal experiments have demonstrated that hyperglycaemia has an initial deleterious effect on yolk sac structure, which then results in embryopathy. Study of the yolk sac by means of ultrasound has not become an important diagnostic method yet. On the basis of clinical studies and animal experiments, however, it may be expected that this organ plays a crucial role in the development of spontaneous abortion and structural congenital defects.