BPM lower each visit?

Cara • Expecting our first baby Jan 1st 2016! Married my best friend after 8 years together 10/10/14.
Each time I see he doctor the beats per minute goes down. I'm 14 weeks 2 days now. Our beats per minute started at 179, then it was 166, then it was 155, and now it is 146. The doctor said it depends on how active the baby is at the visit but he/she seemed the most active at our last visit. Anyone else have this happen??