Ultrasound measured 3 weeks behind

I went in for my 7 week ultrasound and doc said there is only a sac and it measures 4 weeks. I know my last period was Feb 12 and I got a positive pregnancy test before I even missed my period. I’ve had 3 miscarriages before so I wasn’t holding my breath, but the doc wanted me to do a blood check for Beta levels rising. I got the results today and they are rising perfectly normal! I’m shocked! Has this ever happened to anyone else? Did you end up with a healthy baby? Just slow to start or dates wrong? I’d love to hear your stories!