12 week scare

Ni
Went in for my 12 week apt today and she went to go find the heart rate with the dopplar and couldn't find it...so I had to get an sono done to make sure everything was okay and the first thing the doc says is "oh my we have a big baby!" Still took a min to get the heart rate since she couldn't pinpoint it since the little one was moving around so much!! But the heart rate at our 8 week apt was 177 if I remember right and today it's 138. Just wondering if that's common for the heart rate to slow down further along or not?