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Fetal heart rate tends to vary with gestational age in the very early parts of pregnancy. Normal heart rate at 6 weeks is around 90-110 beats per minute (bpm) and at 9 weeks is 140-170 bpm. At 5-8 weeks a bradycardia (less than 90 bpm) is associated with a high risk of miscarriage.
Basically in the sixth week you want to see a heart rate above 100bpm. Your fine, he should never have frightened you like that. If you were a few weeks further along, yes that hr would be considered low. But not at 6 weeks.