please help me!

I had a natural miscarriage very early on back in August. I'm now currently pregnant again and last week went in for a vaginal ultrasound and measured 5 weeks and we only saw a sac. Today I went in again for an ultrasound and the sac measured 6 weeks but still all we could see was the sac. The doctor said this could go either way. So the sac grew exactly how it should but no baby was seen. Any advice or experience with this!? They did take blood so I'll know in a few days I guess.