Babies hearbeat? Why so different?

I've have 3 sonograms so far and I am 11 weeks & 5 days (my doctor is very good and insists on checking the baby if I experience any pain or bleeding. which I was) The first sonogram the babies heartbeat was 156, the second it was 176 and the third it was 136. Is this normal? Why such a huge difference each time? Also, I have had pelvic sonograms each time I have gone, when will I have abdominal scans?