Variations in Heartbeat

Cecilia • Mommy to our rainbow princess Manuela Carolina (born Feb 2017) after a miscarriage in November 2015. Baby #2 due March 2020!
Each time I get an ultrasound they've told me the baby has a different heartbeat rate. 
Today the doctor tried with a Doppler (I'm 12w2d) and couldn't find the heartbeat (talk about having a mini heart attack!). Then she broke out the ultrasound and we were able to see and hear the heartbeat. The rate was 146. Last week at 11w it was 158 (I did have apple juice before the scan) and at 9w3d it was 168. 
Anyone else have these types of variations?