no heartbeat at 6 weeks

Today i went for my first gynae check up, doctor did not find any heart beat through ultrasound and transvaginal ultrasound... doctor ask me to come back in 1-2wks to check up again but he also said to be mentally prepared... any mummy experience this? as i saw on internet alot mummies experience no heartbeat at 5-6 weeks but after they went back again for check up there's a heartbeat...