I think this is true, but I’m hoping one of the others knows why!
For unborn babies and newborns, the heartrate is usually about 120-140 beats per minute. I know this from experience having spent many hours hooked up to a monitor when I was pregnant! My second son was very premature and spent his first month in an incubator attached to lots of machines to monitor his heartrate and again it was mostly in the 120-140 bpm range. It’s much slower for an adult – at rest about 60-100 bpm.