I had an early ultrasound at 5.5 weeks. a yolk sac was detected but not an embryo yet. what are the chances as embryo will develop?

Repeat sono: 5-6 weeks is a little too early to see the fetus. The yolk sac being present is a good sign. You will need to repeat the sonogram in 1-2 weeks for confirmation. Congratulations on the pregnancy.

Very High: I would not be discouraged. That is still very early. The pregnancy changes a lot early in pregnancy. If you repeat next week hopefully you will see the pregnancy itself and the heart beat. A couple days can make a high different between 5 and 6 weeks!