The reason is simple In your example it may be (I don't know much about fetuses), but in general it isn't. The reasons for changes in heart rate variability (H

The reason is simple In your example it may be (I don't know much about fetuses), but in general it isn't. The reasons for changes in heart rate variability (HRV) are generally unknown, although there are various hypotheses and theories. If you read the papers by the company I linked to above that does HRV monitoring for neonatal sepsis, they freely admit that they don't know the mechanism behind the signal they detect. The military figured out that the best soldiers for special forces training had a constant heart rate Interesting. Do you have any links about that?