At 7 weeks pregnant what causes the heartbeat to be low?

It may be normal: In very early pregnancy the low heart rate can be normal. Typically it should increase quickly but many times if you're doing an ultrasound very early that can actually be a normal finding. My recommendation would be to repeat the sonogram later to find out that the heart rate has increased.

Depends.: If your dates have been confirmed by ultrasound, the fetal heart should be beating at >100 beats per minute (bpm). Lower rates are associated with increased risk for miscarriage (the lower the rate the higher the risk). If you are at 5-6 weeks by ultrasound, then even heartbeats at 70 BPM are within normal range and have no adverse implications.