Gestational sac

Is it okay for your gestational sac to measure a few days behind? Today I had an ultrasound and my sac measured 6 weeks and the baby measured 6 weeks 3!days with a heart rate of 121. My doctor said it was perfectly normal and in the range it should be but I’m still a bit worried. Is this okay? Has this ever happened to anyone? This is our rainbow baby after 3 miscarriages so I’m worried about everything.