drop in fetal heart rate?

Since 8 weeks my little one always ranged a heartbeat around 140 bpm. Today at my 31 week appointment my Dr was getting a reading of 120 on the doppler. I asked if that was okay and she said it was. She stated that fetal heart rates range depending on how active they are at the time, and how it also starts to drop closer towards the end of pregnancy. Ofcourse being a ftm I feel a little uneasy that it was lower even though she said it was okay! Anyone else had similar experiences ??