This resource gives info that there is a yolk sac is present, then a viable embryo is also present. A yolk sac is attached to the fetus and is what nourishes it until the placenta take over. In the terms of the link below, it states that a viable embryo is necessary for yolk sac development. Some of the other information at this site we find to not be consistent of what we have seen here. Such as seeing a heartbeat at specific weeks. That is not always the case and often the heartbeat does not show until later although it may be present but not seen on u/s. Over all, there is some good information. Refer to page 13 (page 17 on pdf format).
http://assets.cambridge.org/97805218/25 ... 5290ws.pdf




