First ultrasound measuring behind???

I had my first ultrasound today.. I'm supposed to be 6w3d exactly since we did IVF we know this, but I'm measuring only 5w5d so 5 days behind and we could not see or hear the heartbeat yet. The doctor said it could just be because the embryo took a few days to implant and its behind or it could mean the baby stopped growing and I might miscarry. It's a 50/50 chance at this point it's hard to tell. I am still having pregnancy symptoms and have not had any blood or anything so I'm trying to stay positive. I go back in a week to see how the baby is growing and go from there. Has anyone had anything similar happen to them and had a positive outcome? I'm trying not to worry but it's hard :(