Baby measuring 1 week behind

These last few weeks have just been insane.

My HCG for starters was on the lower end and not doubling like it should. My OB ordered an ultrasound and it only showed a sac measuring 4w6d. I was supposed to be like 6w1d at that time. We thought it was a blighted ovum and so he ordered another ultrasound for today (1 week and 1 day ahead) and this time there was a baby with a heartbeat measuring 6w1d & the heart rate was 122 so a super healthy heart rate. I’m just very shocked and honestly confused cause of all the bad things that have happened to now have a baby with a strong heart beat. Has anyone gone through this? Outcome?

** I’m sure of my Ovulation day as I take OPKs