HELP measuring behind

Measuring at 9weeks 2days at 12 week scan, saw heart beat, is this normal or should I be worried?

i know midwifes add 2 weeks from the first day of your last period is this why I’m dating behind because baby is actually meant to be 10 weeks and I’m just a few days smaller or is something really wrong

Also found out I’m rhesus negative not really been explained to me other than if I don’t get an injection at 28 weeks I could produce antibodies that will attack baby, going for my ‘12’ weeks scan again in 3 weeks bit worried as it’s first pregnancy and many miscarriages in family