The heart starts out at about 85 bpm at about 5 weeks something, although it is usually too early to see it on a scan then.
It increases at 3bpm every day, until it reaches a peak at 9-10 weeks. Then it is usually somewhere in the 170s.
At the 10 week point the brain/central nervous system is well developed enough to be able to regulate the heart rate so it doesn't need to keep speeding up.
It slows down over the next couple of weeks and will eventually reach a stable point around the 140s-150s mark. Just like anyone else though, the baby's heart will beat slower when it is resting and faster if it is active.
So, in short, it is totally normal for it to be slower at 12 weeks than 8 weeks. In fact, that is a very healthy sign. During pregnancy anything between 120-180 can be normal as the baby's heartbeat will fluctuate, and of course all babies are different.