Hello, thanks for posting your query on HCM. A pregnant or gravid uterus contains a gestational sac for a baby. Within it is seen an embryo or the small baby, and another structure attached to it called yolk sac. The yolk sac is not another sac, but a very important structure which facilitates exchange between mother and the developing baby, until the placenta develops. The baby derives nourishment, hematologic, metabolic etc benefits through it. Hence do not worry, as long as there is one yolk sac and baby, you are carrying a single pregnancy. In fact its presence is a reassurance that all seems well. Hope this is helpful. All the best.