Embryo was measuring on target on ultrasound performed at 6 weeks 3 days. Now 8 days later, embryo measures 7 weeks 2 days (rather than 7 weeks 4 days) hearbeat present on both scans. Have had a previous miscarriage and two healthy births, am very worri

No need to worry. The early ultrasound exam of the embryo although fairly accurate still can measure plus or minus a few days. These measurements are small and by just placing the caliber a few tenth of a millimeter more or less will give one more or less day. The simple rule is first trimester scans can be are plus or minus 1 week off, second trimester plus or minus 2 weeks and third plus or minus 3 weeks. .
Why are you having. so many ultrasound scans? Perhaps because your doctor is concerned about something? If there are no other red flags about the health of your fetus, I would not be alarmed simply by the, I presume, CRL (crown-rump length). While this measurement can be quite accurate, there certainly could be ultrasound operator variation which can alter the measurements a bit. Follow recheck advice. Good wishes.