Small Gestational Sac-Advice Needed!

We had our second U/S today. The LMP/EDD date was 9 weeks 1 day. The AUA (Actual Ultrasound Age) was 8 weeks 1 day. The doctor said that everything looked good with the baby. It had a strong heartbeat of 186 and was moving around a lot. However, he was a little concerned with the size of the gestational sac. It was smaller compared to the baby. He is going to check back next week. Has anyone else had this? I am so worried and praying for a healthy baby!