What is the primary function of the yolk sac during early pregnancy?

The yolk sac plays a critical role during early pregnancy, primarily by providing essential nutrients to the developing embryo. During the first few weeks of gestation, before the placenta is fully formed and functional, the yolk sac serves as an important source of nourishment. It is involved in the transfer of nutrients such as proteins and carbohydrates to the embryo from the maternal environment, facilitating early embryonic development.

While the other options describe various functions that are important during pregnancy, none match the primary nutritional role of the yolk sac. For example, while the placenta eventually takes over many of these functions, the yolk sac is crucial prior to its establishment. The implication that the yolk sac serves as a protective barrier highlights its role but does not underscore its primary function, which is nutrition. Similarly, while it is associated with gas exchange and can contribute to this process early on, providing oxygen is not its main purpose. The formation of the placenta is an essential aspect of pregnancy but occurs after the yolk sac has already fulfilled its role in nourishing the developing embryo.