positive bloodwork - negative ultrasound

Lindsay • After 9 miscarriages,we got our little miracle in Feb 2016. My little Aidan! Trying now for baby #2!
Has anyone ever gone through this? I've had positive tests and positive bloodwork from my doc, she sent me for an ultrasound but it showed no sign of pregnancy. I'm currently 5w3d. I would assume the yolk sac could be seen? Feeling quite confused