Is an abdominal ultrasound accurate at 6w4d? could it pick up my heartbeat instead of the baby's? the tech told me the heartbeat was 100 but later the doctor said it was 70? so confused. please help?

The fetal heart (and heartbeat) can usually be detected at a gestational age of 6w4d. Fetal heart rates are usually higher than 100 and certainly 70, so this may be a maternal reading. For assessment of fetal growth, repeat the U/S in a couple weeks, and/or obtain a quantitative HCG determination to help ensure normal progress of fetal growth.

I would call your doctor to confirm this. A fetal HR of 70 is abnormal and I'd be surprised if even a fairly incompetent doctor wasnt concerned about that. If they confirm it was 70, they should have a plan to recheck soon. If they don't, I'd recommend another OB.